Is it bad to think my girlfriend's sister is prettier?

Just keep it to yourself and everyone will be happier.

I'm sure she thinks your brother or friend is hotter than you too.

ask ya gf. she'll tell ya best.

Kinda is

So, what does prettier have to do with a damned thing? Do you believe prettier is nicer, better, or what?

I had that once. I went out with her sister months after breaking up with her.

it's okay. your gf probably knows somebody who's prettier than you too. ;)

Its bad, and to think That is a no win situation