T.F.
It's very, very, very kind of you to offer babysitting services to someone for free. But it's not your baby. She had this baby and it is her responsibility to find care for the child while she works, not yours. You are not doing yourself, this baby, or your teenagers any favors by further injuring yourself in order to make someone else's life easier. Why wouldn't you tell her the truth?? Tell her she needs to find alternative care and then give her a reasonable deadline, like maybe 2 weeks or whatever you are comfortable with. The baby will only get heavier and eventually will become mobile so you will injure yourself even more the longer you wait. Seems to me it's a worse idea to not say anything and then one day you get injured so badly that you can not babysit AT ALL anymore and she'll be stuck scrambling to find another sitter with no notice at all, right?