Fiance's family too overbearing?

Do I have the right to be pissed off that my future in-laws are too overbearing? Amongst other things which have occured, this evening I'm sitting along because my 25 year old fiance has to wait for his 20 year old sister to come home before mid-night while the parents are out. He said he doesn't want to hear it that he's already confronted his parents about it. I don't get it, once we're married next year am I supposed to be ok with him going home to watch his "baby" sister?????? Uggghhhh!!!! His parents say its not right for his 20 year old sister to be alone at night...is there something wrong with me that I don't understand this?