Talk to your cousin. Talk to your cousin at length about this stuff: everything from prescription side-effects to the red tape involved in actually *getting* medicines, to finding work while publicly trans, to just being out in public or having your own place and everything involved with that. I don't like jumping on the "you are a child" bandwagon, but I still remember how much I dreaded telling my mum why she wouldn't be getting any grandchildren from me. Hooray for baptist parents, eh?
I don't remember when news got out that a cousin of mine was intersex, but it was after I'd turned twelve. The family used to have me hang out with her regularly because she didn't have very many friends, but after her intersex condition came to light my part of the family just stopped mentioning her. I didn't pay much attention to it because we weren't very close (from my perspective; if somebody asked her she'd say we were really close), but we compared our respective teenage years and it often looked like we had parallel lives.
I hadn't figured out my own situation until just before my 21st birthday, and I was still sort of living with family except I was the one paying most of the bills while nobody else in the household had a job. I can honestly say it was the worst time of my life, because I had to keep a certain type of 'face' on at all times. The birthday itself was the breaking point, because everyone in the house made it clear that I was being used as a source of income and nothing more. Sure, I got to hear from everyone how much they love me and worry about me and want me to do well in life, constantly, but I've never believed love to be anything more than an emotion people use to manipulated others.
Telling Vs. Not Telling
Telling runs the risk of being tossed out onto the street. Up until I stopped caring about whether the family would have anything to do with me, I kept silent about everything. Nobody but the internet knew anything more than "Fyrra likes to shut herself in her room and play video games all the time when she's not working," and I was fine with that because what everyone didn't know couldn't come bite me in the arse later. I didn't really have friends in person because I was shy back then, and being in The South, I didn't quite have access to people I knew would be accepting. Between the If you turn gay we'll shoot you "jokes" and the constant urge from family members to find somebody to be romantic with so they could have grandchildren, I wouldn't trust them with anything, especially after that one time I brought a slightly overweight nerdy-looking girl home and the instant she was out of the house I was begged not to start dating her.
Contradictory things all over the place. Not telling the family why they needed to stay out of my personal life was an endless source of headaches. There was always pressure from both parents to start dating, the constant proddings over why I was so "secretive" and "why don't you trust us about things that are important to you?" and "you aren't falling in with a bad crowd are you? Because that would be bad." That said, I had nowhere else to go but to other members of the family, because any support network I could have developed back then would have only included other dependent-on-others-for-a-home people.
So yeah, eventually I did the "Hay mom I'm not who you think I am" thing. It was every bit as dramatic and emotional and stupid as you can imagine it was. The "tell your parents because they might surprise you" thing is good, especially if you're lucky enough to have parents who aren't homophobic, and even if you don't you still get that massive burden lifted off of you, but there's more to it than that. I'm not going to claim I know your parents, but I know your situation really well. If your parents are anything like mine, repeating the "god is love" line and saying "we still love you and we want what's best for you" crap, and you want to tell them just to get it over with, there are some things you should prepare for in advance.
1) Always, always, always have a backup plan. In this case, this means "another place to live where they can't interfere." I've got a trans friend who got put into a pray-the-gay-away therapy camp, which I'll consider one of the two worst-case scenarios, with the other being "kicked out onto the street because no son of mine is gonna be a girly queer." They didn't happen with me personally, because I was twenty-one when I came out to the maternal unit and the moment somebody started giving me shit, I packed everything I could after work one morning and moved into a flat I'd leased out.
2) You know this is going to happen, but they are going to pressure you to think that this is just a phase. To them, the gender identity thing is a lot like losing somebody they know and claim to love and having that person replaced with a stranger. The five stages of loss are denial, anger, bargaining, sorrow, and acceptance, and in this case you might never see them reach the last step. Even if you manage to convince them to get you into counseling (often a first step toward acquiring meds for this), they are going to ask you questions and try to milk you for information so they can groom you to see things their way, because parents don't recognize what "patient confidentiality" means when it comes to their own children. They want to know everything, and they will try to pressure for a christian counselor because that is the type of person most likely to mirror their own views on controversial subjects like this one.
3) Be prepared to fight, every step of the way. "I just want to know what's really going on here" is as innocent a way for a casual, friendly conversation to snowball into an argument. All the diagrams and science in the world won't save you from that, and it probably won't grant you any victories, but if you want to press your ground, be sure to have explanations ready for "How can you want a sexchange if you don't even like guys," "how do you know this isn't just a phase," "are you going to chop it off" and possibly "How can you call yourself female if you don't want to have that chopped off" if it applies to you, followed by "But you aren't girly at all! I've known you all your life so I know you inside out!" And then it'll turn into "So where are you going to get the money for all this?" and "You should get a career going before you start this, that way you can be rich and have money to fall back on" which really just means "I want you to put this off for a while and think on it a little longer," which is an arguing point that will never be satisfied because "I want you to think on it a little longer" is an argument people have on vasectomies and tubal ligations. "What if you want to have kids someday" has a similar stalling technique, and at the end of it all, it won't matter because a sufficient enough amount of time will never pass.
Have answers for those questions, especially if somebody starts telling you "you don't want to be a woman because ________." At that point, you can at least be smug and tell them at least you won't have to worry about periods.
You're in for a hell of a trip, regardless of how you go about this. Either way, if you need somebody to blow off steam to, just send a message. I know I'm late with this message, and there's really no right answer, but I'll do what I can to help. After all, it was people on the internet who helped me when I had nobody else~
I still have to call my mum and talk to her *again* about calling me by the wrong name. It's been five @#%&ing years since I started this mess, and I've been moments away from just burning the bridge through pretty much all of it. Don't listen to any of these assholes who might say it's your own fault if you end up unhappy about the way life turns out, though. There's a degree of that that depends on your own input and actions, like not getting caught up in the drug community or prostitution, but other people "telling it like it is" are just being jackasses, regardless of them claiming not to care in the long run, or how much they claim to know about "the real world." Sure, they're right to an extent about the need for thick skin because you -will- be made fun of for being different, but other people being shits isn't your fault.
Hell, look up the name "Leelah Alcorn" and read all the stories on that, maybe try not to spend much time looking at the pictures. This is pretty much the situation we all want you to avoid.