So I'm in 7th grade, and I'm pretty sure I'm in love with my guy best friend. But the problem is that a) he has told me multiple times that he loves me like a sister and b) I'm in a relationship. What should I do?

you have plenty of time in the future for relationships and dating. For now. Focus on school and being a kid.. The person you're dating now most likely won't be the person you will marry. Wait until you get to collage- your relationships will be more serious by that point.. Elementary and high school relationships are mostly kids stuff really.

He likes you as a sister, you do nothing about this guy. If you do not like the guy you are dating and being around him puts you with his best friend all the time, then leave the relationship and get some distance.