Am I being a terrible friend?

Hi everyone… So I have a problem and am not sure what to do about it. I hooked up with my friends younger brother at a Christmas party. We have been staying in touch, even though he doesn’t live in the same country as me. He’s planning on visiting me for his birthday weekend and we’ve both been very honest that we’re looking for a NSA arrangement, sort of friends with benefits who like to travel and party together. Problem is my friend is really not happy and says she “doesn’t see why I need to date her brother. Or the point in us 2 being together”. What do I do?