Yes abusive relationships are hard to leave but she cashed in and dragged the kids out once before. If he is the problem she could have got social services, police, early help all involved and got Marshal to leave so the kids keep their school, bedrooms, home, cats etc
They just moved, spent a load of money, got pregnant all just after one of the kids has been traumatised by a dog bite.
If she moves back after a couple of days again, I'm sorry but social services need to be tougher. The home is unstable for "healthy" children, she has a child with additional needs who needs routine, a child with a genetic issue needing to have a stable go, hospital relationships.
She could have got contraception behind marshals back when she goes out or when she was with her sister and used her as an alibi.