Dear Peter,

It is quite unfortunate that you are going through this. Don't beat yourself too much about it, any one can be a victim of such circumstance. It is safe to assume your brother is having a non approving sexual relationship with your woman.

The real dilemma is not yours but your fiance's. She is the one in the middle of this, the joke is on her!
She loves one man but then his twin is taking advantage of the fact that she can't tell them apart to have non consensual sex with her. This is rape until proven otherwise.

After what I just said, put yourself in her shoes and tell me you would not want justice for such a person regardless of whoever he might be? You have to tell her as soon
as you can, the longer it takes the more difficult it would be to get it behind her.

From your story It seems she was raped brutally and he hurt her so much. She thought you were the one having sex with her and so she put up with it, saving her complains for later. This tells me she is a mature woman who thinks deeply about situations before acting, and of course is willing to make the relationship work. Talk to her and apologise.

As for your brother, ask him firmly but without aggression. The deed has been done,  violence brings nothing good. He needs psychiatric help, it appears like he is going through a rough patch and unconsciously may be pushing away those that really matter to him. In reality, you would forgive him eventually but the scar would always be there especially when you see him.

In the future do not blame your wife for this past experience, she did nothing wrong. Her case is simple: she thought you were being rough on her for some reason and decided to keep it to herself till it was the right time to discuss the issue. If she feels things would not be the same with the relationship (after talking it through), then she may suggest that it is best to go separate ways.  Let her be, if she is yours to keep then she would come back to you.

I wish you luck Peter, yours is a tough one. We will be waiting patiently for feedback from you.