Hmm... That's hard for me to say because I've never had a sibling. But based on the kind of person I am, I'd have to say no. Unless there is just a really strong connection between us, I wouldn't want to cause any kind of awkward dynamics. Imagine if my brother/sister broke up with their girlfriend/boyfriend yet I was still dating their sister. That would just making things awkward and stressful for both families.
Would you date a philosophy major?