Theres a guy who likes me and my sister in the beginning and now likes me(were twins)help!

Im a twin, in the beginning i liked it because me and my friends would hang out with the boy that liked me and my sister, he would flirt with me and my sister and my friends so it wasnt akward. He said i was “more mature than my sister” and im easier to talk to. The boy is a boring kid and is only fun when i have my sister and my friends with me. I couldnt picture myself with him i already told my sister he thinks shes immature but he still likes her as a friend, my sister said she wouldnt talk to him but he told me not to tell my sister.were twins if your a twin you’ll know you cant not tell them. and i just need advise im desperate for anything.