I am sorry that some of the comments have been less than helpful. I get it, it’s human nature to wonder.
I have a cousin, well into her 70s. Sadly, she is now living with dementia, but to my knowledge she has never had a partner or relationship. She was bright, very dry, close to her family and had a very responsible career. Obviously, none of us ever broached the subject, but I think it’s only human nature to wonder. Whenever the issue crossed my brain I would remind myself that it was none of my business and move on mentally. I think the difference was that she had a full and interesting life, so we felt that whatever the situation was, it was probably a conscious choice on her behalf.
From your post I sense that you are concerned about your brother and that is your motivation for asking the question. I think that is a somewhat different situation. It does sound possible that he could be on the autistic spectrum, but there could be so many other things going on for him.
Given that he is very private, I don’t think that it would be a very good idea to broach the subject directly and he is of course, entitled to his privacy anyway.
Perhaps instead you could reach out to him and say something along the lines of, “I know we had a difficult start in life, the older I get, the more I appreciate the importance of those around us. I would love to spend some time with you and get to know you a bit better. I have been in counselling and it’s helped, but it would really help to talk to someone who understood my background.”
It might not work, but it’s worth a try?” Good luck in both your personal journey and in building a better relationship with your brother.