Hi Inforthewin,
.....It sounds all too familiar.
I now have 3gc. Eldest 2 gc b/f is a loser. New partner I have never met.
Makes me wonder what my udd has told this new partner (5yrs r/s)) as none of my family have ever met him and he has never reached out to any of us.
Childcare was always an issue. udd would turn up at my home unannounced just as I was going to bed knowing that I had work saying she was tired. Too tired to stay and spend the night here with gc when offered but not too tired to drive back home to spend the night alone??? I knew that it was an excuse to hook up but at least the gc were safe with me.
Then If udd was not happy with me(often the lack of childcare or not doing something she expected to be done without vocalising it, the jealousy and endlessly reminding me that they are HER children and not mine) it was straight to estrangement.
Once she continued visiting her friend across the street from my home making sure I could see her car and the gc going in and out of the friends home during an estrangement. So cruel.

I think it is only a matter of time before the father and stepmom get a dose of what is going on as so often happens and they will eventually come to you. Afterall you know where they live.
Your udd seems to have landed a good one from a good family. Her partner is in a good job and she is presently well taken care of which is good news and I think it is time to take some financial control back. Chances are your udd will never do any study/ courses while her main focus is only on being with a man 24/7. The same has been for my udd31 who has only ever had 1 paid job that last 2wks before she was sacked because she refused to do the things she was actually employed to do.
As I said on your other post going NC is a personal decision. I think there is often a lot more to consider when gc are on the way because it pulls at our heartstrings but the FOG often keeps us stuck longer than necessary, but NC also allows us to draw a final mark under it when all our boundaries have been crossed and LC has not worked.