Please ANSWER ::::I like my friend's sister, I need advice to see if she likes me?

when it's just you and her just ask Are you interested in me? in a casual way and see how she reacts. if she doesnt like you, she will look genuinely shocked. but if she does she wont make eye contact with you when she responds. so if you want to really know look her in the eyes :) or she might tell you strait out but she seems a little shy for that.