Did I do the right thing in telling (see details)?

My mom (who I live with) is gone for a week on a cruise. My 16 year old little sister asked my dad if she could have a mixed-gender party and he said no since no parents would be there. Well she invited a few guys over, as well as some girls. And while I’d let the no-party rule slide if if was a few of her girl friends, having 16 year old boys over makes me extremely uncomfortable since it explicitly breaks their rules and as the oldest (I’m 20) while my mom is home, if anything were to happen, even if that’s unlikely, it would be my fault. So I called my dad to tell him.

He liives relatively close, and this is one matter I know my not really good at communicating divorced parents agree on. But I still feel so guilty. Should I have kept the sibling loyalty?