My uncle and mum are really close.
I thought they were just friends but one time I went to my moms room to get some clothes out of there and there a lounge room when I walk past and I saw them doing something :s
... But I let it slide and tried to give my mom a look when I saw her the next morning so she knew what she did but I wasn't a 100% sure.
But a couple of months later I went to get some clothes from there again and 100% saw them KISSING and I don't know what to do.. she came into my room 10 minutes later to get my washing but I told her that I saw her and she was pretending she did nothing wrong but she knew she did at the ame time she was admiting it and then she wasn't

What should I please help :( ?

Difficult as it is at your age to form the words to talk with her, you must, because you (and we) don't REALLY know the truth. Each description you have used here is not in any way giving me a feeling that you 'saw' them in any sort of romantic or sexual behavior. The world if full of stories of such misconceptions, the most common one being someone comforting another.

That being said, of course it could be sexual. Sit her down when she isn't busy and (as calmly as you can) tell her that she needs to tell you the truth about her relationship with your uncle - those words. Don't go over every thing you saw or it will bog down. If you think she is lying, just sit there and look in her eyes without a word. Eyes tell a lot. Let her know that this is hurting YOU, the family, and that you can't exist in a home with a dad who is being cheated on. If she still lies, come back here for more advice (from many people) on whether to tell your dad, and how. And please first tell us below how old you are.

Not if she still lies, if you still think she is lying.