OP, my husband has a sibling like this. No interest whatsoever in the family after he left home, came home as little as possible, made it clear that he hated living with them from his early teens, and remained distant. He's now in his late fifties. He's the possessor of five of the six grandchildren and he has not once spent Christmas with his mother in the 20 years I've known him, so she has missed out. He also stayed away when his late father had dementia. I also know other people who have followed the same pattern - just seemed not to want to be around their family at all, left home at 18 and virtually never came back. My friend had a lovely family and she went to uni at the other end of the country, in Edinburgh. That was 33 years ago and she's been home as little as possible. She simply is not interested in her family.
Assuming there's a loving family and no abuse, like your set-up, what it boils down to is that the person is monumentally self-centred. I've noticed a similar pattern in all the people I've observed who were like this when young, where I know the family, know the person, and am pretty sure it's not an abuse situation. Champing at the bit in their teens, secretiveness, and a great distancing when they leave at 18. I'm old enough to see how it's panned out 3 decades later, and they remain utterly selfish towards their families when said parents are old and ill.
So, it might not be anything alarming at all, especially as her brother said she seemed well. She might just be incredibly selfish. I recognise the pattern, to be honest - the never coming home and being always busy, busy, busy. Too busy for the families who love them.
I would adjust your expectations of her right down, or you will continue to get very hurt.