Should you date your best friends sister?

Theres nothing wrong if you date your friends Sister as long as your friend is ok with it. Ask him/her about it, make him/her understand and if he agrees then you are good to go buddy.

How do you tell your best friend you like their brother?

Tell him you think youre starting to grow some feelings for this guy, but you dont want to hurt anyone or make the situation uncomfortable. See how he reacts and just talk openly and honestly with him. You dont need to know exactly how you feel or what you want, just tell him whats going on in your head.

How do I tell my best friend I like her sister?

Just tell your friend you like his sister and ask if hed be uncomfortable if you asked her out. If he says hed be okay with that then you at least have the option to ask her out. If hes not okay with it then you should probably just let it be.