You do not have proof that your Uncle is banging your Mom. It is your presumption. But yes, finding a condom at home, is an indication that your mom is having it out with someone, possibly your Uncle.
Now the crux of the matter. Just because she is parting her legs for her uncle, does not mean that she will do it for you. You have to play it by ear.
I cannot believe that you asked her point blank, that you would like to have sex with her. What did you say “Can I fuck you?” If you did, I would say you are really bold and would not be here, asking us dumb questions. The fact that your mom talks about such intimate matters as her bra (hope you are not imagining this) is an indication that she could get naughty with you.
So, take it further. Ask her if you can buy a proper fitting bra for her? Say you will take the right measurement. There are lots of things you can do, besides just masturbating thinking of her. Think with your head, not with your cock.