My friend has a problem and she needs my help but I don't think my family will be very happy if I help her?

You see , my friend has been asked to go away with her boyfriend for two weeks. Now originally she thought it was going to be just the two of them but it turns out his whole family are going and she has asked me to go with her as support. Normally I would say yes , no question , she is my best friend and I would do anything for her but you see my family has history with her boyfriends family. One of his brothers was engaged to my sister but he cheated on her and broke her heart. My Dad and my friends boyfriends Dad used to work together but there was a massive big argument and my Dad refused to be partners with his Dad and then his Dad took my Dads company out from under him. And my mom hates his mom and it's just one big feud. But my friends boyfriend is a nice guy and everything and I really want to support her but I don't think I can betray my family like that. They told me that they would be very disappointed in me if I went and I too would feel like I wouldn't be welcome in the house of the family that hates my family. I would feel like I would really upset my family if I do go but i'll upset my friend if I don't go. Please help me. What on earth do I do here ? Thanks