Am i wrong ?

My fiancé and i stay with his mom until our house gets ready and his brother recently moved in also. Well his brother doesn’t like me at all and always have something negative to say and my fiancé just let it slide every time. When his brother is around he’s always treating me differently either don’t wanna show affection or constantly yells at me about something. After I told him he treats me differently he goes off and say it’s his brother at the end of the day. But when my family says something out of line to him he wants me to always stick up for him .. I just don’t understand it and I’m pissed !!!