Dear Courtney,
I am sorry for your pain. I wish that I had some magic words of wisdom for you but unfortunately, there are none.
There is no way to force your fiance's family to accept you because the problem is their's not yours. You could be the most perfect soulmate for your fiance but they will not see it because they refuse to see beyond their own prejudices. From what you have said, they have some serious issues themselves (past abuse). They are the ones who are the losers in this case.
You have the choice to accept his family for what they are. Focus on what you and your fiance have together, build and nurture the relationships between your family, fiance, and yourself. Do not define your life by the acceptance or non acceptance of his family. They are not worth it. His family will never be what you imagine. Only if you can accept this, should you go through with your plans to marry. Otherwise, you are going to be faced with continual heartache and rejection.
I have been married for 30 years to my husband. For the first 10 years I frequently cried myself to sleep over the heartache of rejection from his family. It was not until I accepted that his family was not the ideal loving relatives I yearned to have, did I finally have peace in my life and marriage. I expect nothing from them and therefore am never disappointed. I have actually been pleasantly surprised in recent years at a few compliments from my mother-in-law. I do not let these few glimpses of niceness let me forget who they are. I feel sorry that my husband's family missed out on being close to my children and me.
Sorry I did not have anything encouraging for you but I do wish you the best. You sound like a lovely person.