How do you get your girlfriends sister to like you?

Answer 1

Buy her a chocolate mud cake.

Try to have small talks with her, make her laugh and make her get used to being in your presence. When you see her always acknowledge her presence, make sure she knows you understand that if she doesnt want to like you she's not obliged. Many times the sister wont like you because she doesn't trust you, so even if you have an argument with your girlfriend don't say anything that will break her heart or cause her to cry.