I'm sorry you're struggling with your mom, and that you guys have this very big thing between you. It's a really difficult topic to broach.

My concern would be... if your mom hasn't had any therapy or any way to really process this, it might be quite traumatic for her. If you push her to open up to you, it could put her in a really bad place. I don't know - since I don't know you or her. I know that there's stuff in my past that, if pushed by someone (even someone with good intentions) would be too much for me to deal with. I had some very bad experiences with a friend awhile ago who tried to do that with me (in the name of being helpful).

So, I totally understand that you want to know more... and want to understand if what happened affects your relationship with her. But, she might not be able to give you more. I'm sorry for that, it sucks, but that's the way it is. (If she's having other problems, you might gently suggest she try therapy... if she's open to it, I'd imagine that ANY healing she does would help her open up more about her past with you.)

And, I agree... no harm in reaching out to the classmate. Hopefully they can give you some insight into your uncle.

I'm sorry you're dealing with all this... good luck...