I am so sorry this happened to you. My mother was abused by her dad when she was little and I have seen the impact it can have on a person. Try to talk about it, with someone. I recommend counseling even though I know it’s hard. Sometimes you don’t realize the deep roots something like that has in you. You might think you’re ok when you are not really. Please don’t take it to be anything negative, because I really don’t mean it that way. I just want everyone to live the best life they can and knowing the toll your past has had on you can help you in the future.
I have had a little experience in sibling hell, although nowhere near as bad as you. My sister is crazy, I mean completely not normal and delusional about right and wrong, how her actions are perceived, and the impact she has on other people. She has been violent, temperamental, selfish, self-centered, etc etc. She has been diagnosed with ADD, PMDD, and depression and I am certain she has a personality disorder and choses to treat none of them. It was hard, but I completely cut ties with her. I still have to be civil at family functions, but we don’t talk and I have made it abundantly clear, to her and my family, that I don’t want anything to do with her and I don’t want her anywhere near my life. Family, especially parents, have a hard time letting go of the idea of siblings being close, but in this case I say RUN. Seriously your brother is toxic and you can’t let him in your life. What he did is unforgivable, and that fact that he did it and continues to act out in other ways shows that he is beyond reason.
I made it clear that my relationship with my parents is completely separate from the one I don’t have with my sister. Try to talk to your parents when he isn’t around. Schedule the time so that they are receptive and ready for a tough conversation. It seems you are somewhat close to your parents even though they don’t approve of your relationship, but going through something like what you did can make you second guess yourself a lot and prevent you from being strong willed. (no judgement, just what I’ve seen in my mom) This is your life and if you love your Fiance and are 100% sure that you want to marry him, then you need to stick up for that. Talk to them calmly, don’t raise your voice, but be firm in your convictions and feelings. Let them know that you are happy and that you are marrying the man of your dreams, that it is not up for debate even though you want desperately for their approval. That they mean the world to you, but that you are a grown woman with the right to marry whomever you want. Tell them that while you want them to be a part of your wedding, that if they chose not to be you will be sad, but you will still have your wedding. That they are always welcome in your life and that you will always want them to be a part of it, but that they will have to respect the decisions that you make. Be clear that the relationship you and your parents share from here on out is completely and entirely up to them, because there is nothing in your life that you can or will change. Highlight the good aspects of your Fiance and all the reasons they should approve of him and all the reasons why you love him. Tell them that although he might not share their culture (make sure before you say this) he is still willing to participate in it and learn from them. That he wants to be a part of your family and that you want them to accept him. Let them know that you still have your culture and that marrying him doesn’t change that in any way and that they are not losing any part of you, they are just getting a happier more complete you. And that as parents they should want above all else happiness for their children. Don’t let the conversation get away from you. If you see them bringing your brother into it, don’t talk or walk away. Direct the conversation back to where you want it to be. Stay in control or get out and try it another time.
As for your brother, don’t talk to him anymore and try to ignore what he is doing. You already told him where you stand, now stand by it. No contact, period. If he can influence your parents behind your back, then you can influence them behind his. He has nothing to do with their relationship with you and everything you say and do should reflect that fact. It will be hard to transition into a new dynamic, but if you want your parents around you should try. It will be hard for them too, but stay firm in your convictions.
Good luck and sorry for the long message. I just feel for you, I really do.