(It only takes one missed day, or one course of antibiotics for the pill to fail. And these are teenagers we're talking about. If a 14 yo boy can't be relied upon then he shouldn't assume a 14 yo girl can either, and to the same extent. I lecture DS in the car so he can't escape. The DDs too)
I'm not sure having the DSD spend time with babies or other mothers would have the effect of making her think twice. She's determined to have the baby and seeing others with theirs will probably reinforce her desire.
I don't think anyone, no matter what age or how mature they are, has any idea what they are in for before their first baby arrives. You can read books and try to prepare, but you can only really experience the full-on mother and baby life when you're in the thick of it, and the only thing sustaining you is the fact that there's no putting the baby back where it came from. Most mothers deal with it and deal with it and deal with it, more or less successfully, learning and maturing a lot as they go along.
Duchesse, I think your earlier post has a lot of sense in it. Tess seems to me to be a perfect 'caretaker' for this whole family, with the DH even alluding to it (shamefully) when he accused her of mollycoddling the boys.