I have a close relative in her mid-teens, about 20 years younger than me. I looked after her a lot when she was small and our relationship was close and loving. She's always been very shy and sensitive but I poured a lot of time and love and 1:1 attention into her and accepted this was just how she was. We are on the whole a family of quiet introverts - and I now suspect quite a bit of ASD - so it wasn't an issue. I was always happy to put in the time to coax her out of herself.
The problem now is that she seems to have settled into her shyness, to the point that she really doesn't speak or engage at all if she can help it. She first met my partner over ten years ago, and she has resolutely never spoken to him. Genuinely. He's tried so hard over the years to build a relationship and she continues to act as if he isn't even in the room. She does this with my brother's partner too. She managed it with at least one of her grandparents (I don't know the other side of the family well enough to comment) until their death. It's one thing for a four-year-old to be quiet and shy at first meeting but when it's been going on into GCSE age it starts to get weird. She's never been encouraged to engage and her parents tend to speak for her. I've gently tried to broach it with them but they are very much 'this is how she is/she's young for her age' and have volunteered without my asking that they feel she is NT.
From the age of about 8 she's always just brought a tablet or a book along to get-togethers and sat in the corner totally disengaged from everything. If she stays at our house she just hides in her room, especially if my partner's about. We'd love to include her but she either doesn't know how to take part or really doesn't want to. Given that I've been told she is NT I don't feel I can treat/think of her as ND, so I struggle to know how to proceed without interpreting her behaviour as rudeness. I'm upset that I put so much energy into trying to maintain a relationship with her when she won't even acknowledge the other people in my immediate family. FWIW it's hard even for me to draw her out these days. I'd love to have a positive relationship with her as she enters adulthood but it feels futile and has done for quite some time.
I don't think this is personal - in that she has few friends and doesn't engage in extracurricular stuff, clubs activities... she either prefers to be solitary or knows no other way. But it makes me sad. I think she might be surprised and upset if we were to drift apart but I'm struggling to know how to go on.