OP,
Bottom line is you have been sucked into a carer skivvy au pair situation and your partner will put him and his children first to protect the status quo.
You think your MH is bad now?
Imagine what it is going to be like when you stay and your daughter is so damaged and starts to act out, and you realise your choice to put him and HIS daughters first has had a catastrophic affect on your daughter?
When she starts asking you why you failed her, put others first, didn't protect her, and when she judges you really really harshly?
How will you feel then?
Because that's whats ahead of you.
Be brave.
Start planning.
Put your child first.
Your daughter does not deserve this.
A mother whose choices have led to her MH being in bits.
Your daughter deserves her mother well.
Your sole responsibility is to your child, not his children.
Expect push back from him.
Men hate loosing free childcare.
The posts are not harsh.
They are advocating for your daughter because you are not.
Do the right thing by YOUR child.