Keep in mind that people come and go from our lives, but family is forever. Don't ruin your relationship with your sister for some stupid boy. My and my sister's motto is "A sister is a forever friend."
Sisters over misters! Lol :) I wouldn't risk my relationship with my sister over some guy. It's not worth the worry, arguments, and pain. I'd try forget about him on both sides. That's the fair thing to do. There will be plenty of other guys out there!