Yes it is and you know it or you wouldn't ask.
Men fantasize, yes, but when it's about your girlfriend's younger sister, then there is seriously something wrong in your relationship.
No. Its perfectly normal to fantasize. Enjoy it!
No, you are wrong.
no this is not weird. its actually more common then people think.
Yes.
no it is not wrong
Should stay sister like otherwise it would be wrong.
yes
Fantasize
kinda
She doesn't have a twin sister she has 2 older sister's one is 5 yrs older the other is 7 yrs older! And there isn't anything wrong with them either.
Never spy on your nude sister in the bathroom. This is morally wrong. Spying on someone is also wrong morally and legally.