I want to touch my girlfriend's brother's fiancee's ear
Are you attracted to ears?
I can't help myself from being attracted by her ears.
Everyone has their kinks, but you shouldn't indulge in yours if it involves crossing someone's personal space. If you act on your feelings without permission you may make them uncomfortable. They may try to avoid you in the future.
It's only my girlfriend's brother's fiancee, no big deal and also I can try to make her uncomfortable.
If you make her uncomfortable she might become upset. You're better off getting permission instead of crossing any boundaries. They'll be happy you asked for consent before doing whatever you want.