sounds like drama! I think that your mom and boyfriend are doing the wrong thing and should tell you the truth, if they want to be together, fine, but the least your bf could do would be to break up with you. I wouldn't advice you to sleep with his uncle, two wrongs do not make one right, and you need to show him that you are at a higher level than him. Confront him and tell him that it's over between you two. But don't count on any changes, he may still be with your mom and you must accept it somehow.. ok there might no evidence, but isn't it enough evidence that they spend much time together? I can hardly imagine them as best friends. Especially if your family, more specifically your parents, are not so close as before, and things are not as they used to be, people don't think about boundaries and what's right and wrong, they think 'I only live once' and take every chance that can lead to happiness, even it means taking risks. And as you say, your dad is way too busy with work, probably has no time for his wife, and therefore it's not impossible that she seeks attention from your bf.
Talk to your mother first and try to get some awnsers. A cheating boyfriend can come and go but if your mother is really involved this could effect you for the rest of your life. If it turns out to be true, Convince her or tell your father about it.
Talk to your boyfriend if the mom chat isn't working. If he fusses up your parents and you and your boyfriend should all have a big conversation about this since you cant keep the 4 like this together.
Don't cheat or something with your uncle. That is not going to work since if he did cheated on you it means he really doesn;t care about you romanticly anymore. Plus its not in your best intrest.
I sort of have this intention to start up with him something as a revenge and so that he sees what hes doing to me.
There are 2 very wrong things with that:
1. It's implying you actually WANT to be with someone who would cheat on you... with your mother. 2. You consider cheating on you with family is bad, and you would do the same.
Here's my answer:
It's not bad it's UNBELIEVABLE. Check whether it's true or not, and act appropriately, which means dumping him and telling your father. If you're young and easily influenced, know that their mentality if it really is happening, is not normal, nor acceptable. Don't consider that because you saw 2 people close to you do it it's anything less than abominable and a very rare case. Doing something as major as being with your uncle or hitting on to appeal to someone that disgusting is only something you'd regret deeply when you're able to comprehend the gravity of it later.
Talk to the people involved. Everything might be a big misunderstanding.
If not - break up with your boyfriend and break the contact with your mother. He has cheated and she has broken a whole lot of trust that should be in any healthy parent - child-relation. They both obviously don't care about you if they can't at least work out the situation before they got at it. Really,
Though, reading your solution to it all, you sound really stupid so I guess you could just stay in your weird relationship sharing boyfriend with your mother while sleeping with his uncle... really, this is starting to sound like a bad soup-opera. Are you sure you're not bored and trolling?