I couldn't date Pippa, despite all her advances on me
This one is simple, if a girl has a sister who I find more attractive than said girl, I can't date the girl.
Even if they are both cute, as long as one is appreciably cuter, I will not date the uglier one. Back home I very close to a family with three daughters - one around my age, one older, and one younger. All three are adorable. Unfortunately the middle one (my age range) is the least pretty, but still very pretty. The oldest one is married, and the youngest one is too young for me to ever consider dating, but I just couldn't go after the middle one.
Sisters usually look similar, and the last thing I want to deal with every Thanksgiving and Christmas is a better looking version of my wife running around taunting me. No thanks.