Period started at and early age?!? Help!!

Desiree
Ok so my step daughter is 8 years old. Just turned eight in April.  She started her period today. Is it normal for a girl her age to start that young? I've heard of girls starting that early before. We've called her physician and we're waiting on a call back. Please tell me I'm not the only one who thinks she may be an early starter. Her sister is 11 and hasn't started yet but we have had the period talk with them both. No rude comments please. I don't have children of my own so parenting is still somewhat new to me. We have been in each others lives for around 8ish or 9ish months and have been living together for 6. We provide them with everything they need and we try our best to ease their fears of the changes there bodies are going through but two people have said that's not right and now it has her a little freaked out. Wasn't sure where to post this so sorry if it's not in the right place:/ I have this posted in another topic column as well.