Why would your financial stability be ruined by your stepdaughters? Are you supporting them?
If the babysitting is stressing you out - don't do it. Nothing says you have to babysit for them, correct? If your hubby wants to, let him, but find somewhere else to be when the grands are over.
You cannot change the past and the way your hubby and his Ex raised their daughter. I think your hubby's response of "what's done is done" is very telling of the way these girls were raised. But, that is the past.
You don't mention what kind of relationship you have with the girls - but have you tried to engage them in discussions about their plans for their children, themselves, higher education, financial planning, etc.
They are adults now. They are responsible for their own decisions.
I get the frustration over "watching" the girls have babies and live life with seeming irresponsibility. But either you take steps to help change them, or you have to let it go - for your own sanity.
Good Luck
God Bless