I understand your concern about her getting pregnant, however, it would do no good to say anything to her. I learned with my own kids, and other family members, that it doesn't really put a "watch" over their actions whenever we tell them something they already know.
When my younger sibling was doing things that drove me crazy, I thought that I could talk to her and it would stop her from the things she was doing. This was after we were grown. One day my father told me that I had to learn that I could not "control" my sister's life. It shocked me! I wasn't trying to control her,I was trying to help her from making a terrible mistake. If she would go by the advice I gave her....it would make her life so much better. So, see,I could not see it in myself as trying to control b/c I loved her. But it was a type of control and you want to protect your W from maKing things more complicated and if she would listen to your caution about getting pregnant.......but sweetie she "knows" that already and it won't stop her careless ways for you to say anything. If anything, she might do something out of "spite" toward you.
I am still dealing with other family members who I want to tell them what they need to do or don't do (lol) but I hope I've learned when to keep my mouth closed. One thing I've noticed.......is how few actually "ask" for advice.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!