This is a confusing question because, if you're a girl, then you are yourself your sister's sister. Would you be family?
Or could you be talking about a half sister?
Or something else? For instance, in MASH, Father Mulcahy's sister was a nun and he sometimes referred to her as, "My sister, the Sister." So if your sister's sister was a religious sister or a nursing sister she may be family, or she may not. Depending on the type of sisterhood. :)
Your sister's sister is your sister, of course. Because if it's your sister's sister, your sister's sister is the sister of your sister, and since she is the sister of your sister she-- never mind, that didn't come out the way I meant it to.
Your sister's sister is not related to you, she's related to your brother who is related to your sister and their all related to your parents, who are not related to you. I doubt any of them consider you family.