@Arius LaVari: I mean. Any -sibling- of Leontine and Colm might be drug into the whole "lover" category at the same time. With the previous life it would be safer to assume that she would have been an understudy or assistant of his that was like an adopted daughter. He never actually had physical relations with people because he thought of them as a disease.
So, My counter offers aren't too far off from your original ideas:
Understudy in Life One, Mainly a protected Adopted daughter under his protection
If you are a sibling in life 2, it's going to be a possible sibling/lover situation, because they will actively try to seduce their other sister. Their view of siblings is very different than what is conventional.