Jesus, you trust him after what he has just done to you, no, no, no, he has to now earn that trust back, he would most certainly have cheated on you and with his friend's sister, that's lovely, both talking about you behind your back; him and his mate sound delightful.
You are being a complete pushover, sorry if you don't want to hear that; he was actively seeking either sex or a relationship start up with someone else whilst being with you, of course he would have went through with it, wise up.
Why does he want to go, he should be fucken mortified at his behaviour and out of respect for you, declining the invite; at least until all this has actually blown over which could take years!
You are going to a wedding where him, his mate and his sister were actively going behind your back to betray you and take the piss, nah, sorry, you're going to go and put yourself through that torture for what, to save his fucken face?
Sorry to sound harsh but honestly, you are just playing along with him and it's all going in his favour, he gets to have no favours at the moment, he's lucky you stayed with him, you got that right.