Cousin troubles-HELP!!!!?

I'm 13. I have 5 cousins. 'Ryan' is 15 and 'Nathan' is 12. We go on a grandparent trip every year. Ryan and I get along wonderfully, and so do Nate and I when we're alone. But when we are with Ryan whenever I open my mouth Nate always has a degrading sarcastic comment for me and normally I just ignore him but sometimes I retaliate which gets us into a huge fight and then Ryan tries to break us up by saying hey Kasa stop causing all the fights all the time. What should I do to make us get along better?