What's the sister's obligation?

I recently watched a program about a mother with a kidnapped daughter living with her father in Greece. There's more to it, as custody and kidnapping cases tend to go, but the relevant part for me is the fact that the daughter has not only expressed disinterest in returning to her mother, but expressed a desire to visit her older full sister. The father is accused of being a child rapist, his children being the victims. What I wonder is if it would be right for the older sister to seek a relationship with her kidnapped sister when she rejects their mother. I would think the mother wouldn't want to stand in the way of a relationship between them, but should that matter? Could you form a relationship with a sibling that hates your mutual mother?

Note that the last update on this case, that I saw, as part of the program, was from 2005.