I called his sister a bitch

Last night my boyfriend sister gave birth we took a five hour trip up to see here and the baby , he then had a fight with his other sister (and we were supposed to stay with here in the same flat that night ,but no here flat ) so she pretty much told us to fuck off and get a hotel room witch we did . This was at midnight after a whole working day and a five hour drive . I can’t justify this behavior towards us no matter how bad the fight was . The next day when we went to the hospital we had to act like nothing happened, we paid for here breakfast, and drove here around . When we started to head back home , and yes I might have not been a pure joy during the day but I still treated everyone with respect, I told him in the car that his sister acted like a bitch and he had a full on lash at me that his family comes before everything. Right now we’re in the car and I’m not taking to him . I think we’re at a point where we’re living together and talking about getting married and I’m the one who takes care of him that I should come first place just like family . But he was pretty mean to me . What should be my next step ?