MOTHER IN LAW PROBS

Please tell me if I'm over reacting. My fiancé & I are eloping to SOCAL where his parents live on March 12th. We fly in on the Wednesday before, as do his sister and her husband. They live in Washington, and we live in the Midwest. His sister & her family are staying with his mother in her small home, and offered for them to stay there before asking us. There isn't enough room for us all so we opted for a hotel. Then while discussing a rental car my fiancé tells me that we can just have his sister pick us up from the airport & take us to pick up our rental car since it is cheaper when you're not at the airport. Curious, i asked if they were renting a car? He said "no, mom is letting them use her car while they are there." Ok, so we have to pay for a hotel & rental car while we are there for our wedding, but his sister gets an all expense paid vacation & we get nothing? (She paid for their flights there to stay for 10 days) It seems like she favors his sister over everything because she is incredibly dependent on her & im about to freak. 🙄 She tries to suck up to me because she knows it makes me angry but I can't help but be annoyed. Parents shouldn't favor one child over the other.