I don't have kids myself yet, so I don't know what I would do in that situation personally, but I don't see anything wrong with it. As PP have mentioned, if it has been the sex talk I would be upset about it, but periods, no.
Another way you could think about it - your dd asked your step-daughter in law a question, which she answered honestly. I think this is a good thing, as it gives your dd someone else she can talk to and trust. There are some things people just don't want to talk to their mums about! When I first got my period, I was home with my dad and my sister. I didn't tell mum (although I'm sure my dad did) until the next one or so. Why? Because I didn't want the embarrassing gushing and tears over "becoming a woman" that got heaped on my sister.
I would be happy to know that my daughter had someone else she could trust to give her accurate information about things that relate to growing up. Sure, I would mention to your stepsons wife that you want to be the one to give the sex talk to your dd, but also thank her for giving your dd accurate information and also for being someone your dd can trust. At this point you can ask her to refer all sex questions etc to you, but if dd has anything she wants to talk about but not to you, then she has someone she can talk to openly and honestly. I also think this will open the gates for communication, because often these things are talked about behind closed doors like taboo topics, when really they are everyday things that happen to what, 50% of the entire population. If your dd knows she can have open communication about these things, chances are you'll know about it as soon as it happens because your dd will want to tell you and talk to you about it.