@Tinkobell
I totally understand exactly how you feel. I have a sibling who exhibits the behaviours and traits you describe for your SIL, even down to the personal relationships and we have wondered about her sexuality.
(Note - there is nothing wrong with them being gay at all - just mentioning it, as all we would like is for them to settle down, but with no relationships to speak off - its hard to know how that will happen).
My sister has made some very poor decisions and is a people pleaser so easily manipulated, to the point she has signed up to loans & phone contracts in her name for other people and ended up in court after doing something a "friend" told her to do.
She is very bad with money also, and often asks for a loan until payday to top up the car with fuel, but will then drive people all over the place to help them out.
Anyway - I am nearly 10 years older than my sister, and could see that she was struggling in senior school and suggested back then that there could be something else going on. Parents were dismissive, and because she was a quiet but poor student, she was just labelled as nice, but a bit slow.
Her late 20s was a really tough time for my sister, and having moved out, she was being heavily controlled by flatmates, who were taking all her food and money. They were trying to ostracise her from us and it was very difficult, she cancelled visits, and kept secrets and hardly saw and spoke to any of us. It took a friend driving her to my parents to tell them of the situation for things to change. She moved back home to my parents house, and I went with her to collect her things the same day.
It was at that point we were finally able to talk to her, with this friend present, so that she listened. Every conversation prior, for years, had been dismissed and ignored, this time, the friend was saying the same we had been saying (that she needed help with decision making, that she needed support managing money, that her judge of character wasn't great etc), and it finally registered. She has an awfully hot temper, and has squared up to me many times in an argument (often out of nowhere!), so it was suggested that she seek help for that, and also discuss depression. She saw a councillor and things improved a little. Then as she was receptive, I suggested that she could be on the autistic spectrum, and asked her to think about talking to her doctor, which she did. Took 2 years from then but she did get an Aspergers diagnosis - high functioning, as she can keep a job (same job for over 10yrs), but socially, she struggles so much.
Like you said, a late diagnosis worried her, and us, as to the effect it could have on her mental wellbeing, it was already delicate, but in fact it has had the opposite affect. She understands now why she feels different, and didn't get the social prompts that other people understand. We now understand her properly, and able to bare in mind how she looks at things so family life is less fraught. Having a reason for her different behaviours and actions have given her such a calming affect, she feels so less confused.
Like you, my DH and I are worried for what will happen when my parents are no longer with us. She is living with a friend now, but that will naturally come to an end when the friend wants to settle down, & there is no way she can afford to live on her own (and I worry about her being on her own!) We have a child with ADHD and a chronic health condition, so have our hands full at home, and worry for their future!
I have said that I understand if parents give a greater inheritance to her as she will need it more that us. But, I can't be there to pick up the pieces all the time, I have so much I'm dealing with already. I too have lost sleep over the future.
Sorry this has become so long! there is so much to consider when you have a family member like this, they need some support to be independent from family, but just a little bit, and there is just nothing out there to help with having this full independence. You don't want to be too involved - so giving them independence, but the second someone isn't helping it falls apart for them! They are grown adults but need help in just a few aspects- and its hard!
Happy to talk via PM if it helps!