L.M.
I agree with the others, normal is a very broad range. Typically, puberty is coming earlier than it used to but 13 is still normal. Additionally, I understand that many times girls that are very athletic may be even later than that.
I also agree what MIL needs to mind her own business and stop discussing personal details with the family. I have a relative like that and that is why I share nothing I don't want repeated. How does MIL know that she hasn't started her period yet? If mom or dad is sharing, they need to STOP IT NOW!