I think that your son probably felt his hormones and, like most males i know, thought that the female relative would probably feel the same. So he broached the subject and his advances were declined.
I get that she was traumatised. She’s young and he should not be asking a minor. He was completely out of order. But I’m glad she had the strength to say no. That she didn’t feel obliged to say yes.
And also that he didnt try anything without her permission. However he is being treated as though he did. As though he tried it in with her and she slapped his hand away. Or he raped her.
Seriously, if he had would he be treated any worse? He’s being treated like a rapist by everyone. While there needs to be consequences as his error was serious, this treatment is a step too far.
His own mother despised him. His family have rejected him. He no longer feels any family love at all and is losing weight.
It seems like an extreme response. Yes he should have had a bollocking for it as she is under age and he was totally wrong and inappropriate. But not the extent his is an outcast.
His own mother should be putting the rest of the family right. Telling them that she abhors his behaviour that day and he has had a good talking to and counselling and understands what he did wrong but he is still a part of this family and he will never ever make this mistake again and has apologised and attended sessions to understand why his behaviour was wrong and is still part of this family and will be attending future family events with you.
Perhaps you need counselling too to get over this as it’s obviously very upsetting. But it’s not the end of the world.