Wow, that is early! But sounds like you are handling it well, and your daughter's attitude will make it easier for you.
I have heard that earlier onset of menses is a risk factor for BC later in life. The more times a woman menstruates, the more times ovulation occurs...or something like that. I have heard that is one reason that a woman who has never been PG is at higher risk...since we stop for 9 months during pregnancy, and if there are more than one pregnancy, then that is a couple years less of having periods and fewer times to ovulate.
I have heard theories that our American diet contributes to the earlier development, with more fat and unhealthy diets....you might consider talking to her pediatrician to be sure he thinks everything is ok. THere are psychological implications that will come with this for her...being the first to develop, first to mature, etc. And the timing with your BC, she may have fears of getting older or developing further, fearing that she too will get BC. I think that they can even do treatments to delay or postpone menstruation in young girls, but I do not know much about it...and certainly I am not endorsing it,...just encouraging you to check your options for her sake.
But also, remember this is a beautiful event, as your daughter is becoming a little adult...