My fiancé’s sister treats me nastily

My fiancé's sister treats me nastily. Once she even told us to get out of their house (via text message). Since then, I've told my bf that I will never go there again, because I won't be in a place where they clearly don't want me. She asks him to take care of her dog on Sunday. And he, asks if he can take care of it with me. To which she says "it's all the same to me, I won't be here anyway". Can you imagine that? And he explains to me so delighted that An doesn't mind! I immediately imagined a scene in which An, however, arrives faster and makes the scene of the year in which she yells that I should get out of her house because she doesn't want to see me. I refused firmly. At the same time, I felt like he was spitting in my face. As if my beliefs and resentment were not important at all. That if his sister agreed then I should say "oh my, great. Of course I'll come with you to watch her doggie!". I feel like his family still comes first and the fact that his sister treats me like garbage doesn't change anything at all in their relationship. I'm even starting to think about breaking off the engagement, because I think it might be the same after the wedding. What would you do if you were me?