Brother-sister halloween costume ideas?

I’m going out to visit my little sis in San Francisco for halloween and I was wondering if you guys had any suggestions for a brother-sister costume. Her boyfriend will be partying with us that night too, so ideally it would be something that could include him too.

Also I’m going to be on a plane so nothing too expensive/elaborate, but I was hoping you guys could help me think of something clever or fun. Thanks guys.