A huge thing has happened to her and she is adjusting (albeit not much at the moment)
A huge thing has happened to you and you've had to step right up and fill her shoes as well as yours. Your DD is , frankly, lucky that she's had that as an option, but it has to be addressed.
It's not sustainable or fair on anyone. Not on you, not on DGS who's primary carer should be his Mum, and also not on your DD who isn't going to be adapting and adjusting the longer she leaves it.
I would suggest an agreed arrangement. Something like:
Ok DD. I will babysit two nights a week. One week night and one weekend. On those nights you go out, let off steam etc. BUT we agree those nights in advance so i can plan to babysit, and on the weeknight, you are home at a time and in a state that means you get up in the morning with the baby.
For now, I will give you your weekend night and following morning to sleep it off.
You also have your phone on and are contactable, You are a mother. I won't ring you unless it's an emergency, but if I do, I absolutely expect to hear back from you within the hour.
Be clear that the other five nights are not an option.
Be clear that if she abuses your offer, it gets withdrawn
Then after a month, reduce to one night a week and no morning support
An so on.
Do you think that could work?