Need step daughter advice on women stuff...

Tl/dr at bottom.So, to start out I am a 33f, engaged to a 35m. Along with that package came his 2 children, a boy, and a 16 year old daughter. They live with us, along with my 2 young sons.I have no experience with a daughter, or a teen girl. I was one, several years ago, but I was a very laid back teen and my mom never had "the talk" with me. Didn't go to a gyno until I was married and pregnant with my first son.So, the story begins, several weeks back my step daughter, out of the blue informs me she will probably have to go on birth control. She lives with us, and her dad keeps a pretty tight leash on her so I am utterly shocked. I asked why and she fed me a long story about how since she started her period at 11, she will have 2 periods a month that both last 10 to 12 days each. I'm not doctor, but I know this isn't normal and can't be healthy. I asked if she had told her mom. She said yes. I asked if mom was concerned, she said no. Her mom hadn't taken her to a gyno and had never brought it up to her pediatrician.This, to me, seems a little off. If she had never been to a doctor, how would she know that she needs birth control or that birth control can help regulate your periods. Regardless, I tell her at 16 she should go see a gynecology, if there is a problem, it should be checked out. I talked it over with my fiancé and made her and I both appointments with my gyno, (I was due for my yearly anyway).So, in the mean time, I notice, by the bathroom trash can, that her story isn't adding up about menstruating 20-25 days a month. (I buy her tampons, so that doesn't add up as well).The other, I guess "red flag" in her story, she was adamant that her mom not know about her appointment.By this point I'm guessing she either already is, or is planning to become sexually active and wants to be on birth control. Her dad is totally against it. Says no 16 year old daughter of his will be put on birth control. (The daughter is the type who would brag about it and if mom found out would use it against us).So, what do I do? I don't want her to end up pregnant. I'm definitely not raising another baby. I want her to feel comfortable coming to me with this stuff, but also don't want her to think I'm a push over and believed her story. Also, if her dad doesn't okay the b.c. then I think she'll end up having sex anyway...Also cross posting this in r/askwomenadviceTl/dr.... step daughter made up elaborate lie to get on birth control, dad isn't going to okay it. What do I do? I don't want to be a grandma. via /r/Parenting http://ift.tt/2pzE7Nt