Really like my sisters friend,but every time I'm with her so is my sis,how do I ask this girl out
Talk to your sister and ask her to help you out. She can organize for you to meet up with the girl alone and then you can ask her out. You can also ask her in front of her sister.
Tell your sister what you want to do and swear her to secrecy. Just ask her out when your sister is not around.
Ask your sister to help you and give you advise...then find a way to tell your sister to leave you and her friend alone