First, I want to say what a wonderful thing it is I think you are doing. I am so sorry the time with your wife was so short. Please don't take what I am going to say as a dismissal of your capabilities as a good parent. Good parents sometimes need to recognize their limitations.
You are in an extraordinarily unique situation. It is virtually impossible for you to have given birth to your stepdaughter as you are too close in age. So in terms of age you are more like an older brother. You are also close enough in age that she could be sexually attracted to your friends or even to you.
You are also not sexually active now or ever and it is really hard to answer questions that you only know second hand the answers to.
So for all these reasons I think you really need to get the help of someone else to answer her questions. She really needs to talk to a person who does in fact have experience with sex and is female and is older.
I'm assuming she is menstruating, but if she is not, that is likely to start very soon. In fact asking this question indicates the hormones are working normally.
First, I would go to a sister of your own, a grandmother, an aunt, or great aunt - someone who she would feel comfortable talking with and you think would have values similar to your own. If there is no one like that, then think about the things she does - is there a favorite teacher, a school counselor, a coach?
If there really isn't any woman in your life that fits the bill, I suggest you go to Planned Parenthood and talk to a counselor there. They may have a counselor on staff that could help, but if not, they will have materials to help. They may have videos you can borrow - for you or for her. They definitely will have pamplets that discuss different issues.
Mostly when kids first start to ask questiosn about sex they don't actually want a lot of information. But most kids start asking questions earlier than 13 - so she may have some more involved questions.
If in the end you are the one who ends up discussing this with I suggest you do it at a time where other activities are going on. Talk while preparing or eating dinner, while driving a long distance in the car, while playing a card game - this is to help keep a comfortable distance when talking about something that can be very embarrassing. It allows time for silence to think which doesn't seem so weird if something else is going on.
Explore this site. It was designed for people with questions. There are hundreds of pages besides this one. You might want to start with
http://www.sexsupport.org/curious.html.
Good luck. Feel free to come back and ask as many questions as you like. You can also email me at susan@sexxsupport.org
Susan