lol... if they become half-sisters they'll probably find it very difficult spending all their time with each other; so they'll probably need different friends I would think.
But if they seem to still determined to spend every moment with each other; you have two choices... tell them straight and ask them if they still want to be friends with you and still hang around with you. Or ditch them.... you can find new friends who wont treat you like the black sheep. 😄