depends
either lie "i like you, but im not ready for a relationship right now, maybe in a year?" they will most likely move on
or be honest "look, i just cant see you that way, your my friends sibling, not my love."
How do I tell a middle schooler who confessed to me that I don’t feel the same way (also I told them I like them too bc they have really bad social anxiety and I didn’t want to make it worse) also they keep me “cutie” and they get rly anxious if I don’t text them