I have very mixed feelings about this. On the one hand I really feel for the adult half-sister given the situation all together. I can understand why she would be very resistant to taking on that 7-year-old, ALONG WITH HER BROTHERS FFS. Sure the three probably should not be separated under these circumstances, and a separation after the deaths of her parents would be extremely hard for that little girl to cope with.
But there's a familial connection to ONLY the little girl.
On the other hand, I really identify with the lost and lonely little 7-year-old girl in me who, tho I had two parents and 4 siblings, was profoundly socially isolated because of all the things that led up to a suicide attempt at age 27. That poor little girl. To lose all that and have grandparents trying to get rid of her, too...
I would hope that at the very least the adult sister would reach out to the little girl and offer sisterhood, with occasional meetings and outings so that the little girl would know she has ONE adult family member who truly cares about her. I understand not wanting to take on the full responsibilities of parenthood for a child she's never met before. But I have a really hard time thinking it would be ok for her to totally abandon the little girl.
Yeah, I know...having that little girl in her life would mean that her own grandparents would likely be even more constantly threatening and gaslighting her.
But that little girl did nothing to create the situation. And someone in the family needs to truly care about her. At least to offer a shoulder to cry on and a big sister to hear the pain that she feels. Maybe take her out to the zoo occasionally, or to get an ice cream cone, or do some such other thing to form a bond so the little girl knows that someone believes SHE MATTERS.
It's not about any kind of "obligation." It's about caring about a lost little girl with whom one does in fact share a familial connection even if it's never yet been an in person one.
Also the dad's family should be willing to throw in financial support!