Should this be a matter of concern?

By all means take it to your stepfather. There's no reason at all a seven year old girl should be talking about, let alone knowing anything about, making out with a boy. Your stepfather has a right to know what his daughter is doing and saying. It's entirely possible that her mother doesn't even know what she's hearing from these girls, and if she lives with her mother most of the time, then she definetly needs to know.