To add, this is a pattern I've seen more than once:
You have a family member who, for whatever reason, is more "obviously" autistic than you are.
The relationship between you isn't great, maybe directly related to them being visibly autistic and triggering your self judgement, maybe for unrelated reasons.
This makes it much more difficult to accept that you might also be autistic. Because you don't want to have anything in common with them.
This is especially true if you have a lot of internalised abelism and struggle to be compassionate towards yourself, and also if you've been judgemental towards that person.
Now you not only have to acknowledge that you may, in fact, have something in common, but that you've been as unkind to them, as you're being to yourself.