My fiance's family doesn't like me. It's extreme at this point! what to do?

So I am getting married in September. My fiances family doesn't like me at all. His mother and I are sort of on good terms but his sister won't even make an effort. She gets so jealous and mad when i come to town that she has gone to extremes to put herself in the hospital to get all the attention. Which i don't mind that she wants attention but seriously this is ridiculous. I want to get married and get along with my in laws. What should i do or say?
My fiance's family doesn't like me. It's extreme at this point! what to do?
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