It's just not as easy as that, force him to pay....... the court will take into account a lot of factors. Maintenance that she will be awarded includes everything you cant get extra money to pay the mortgage. Like you said if he walks away from the mortgage ( he cant actually walk away he signed a contract with the bank ) but if he chooses that he doesn't want the house anymore he has the right to sell the house. The only way your sister can stop this is if she proved to the bank that she could repay the motrgage herself ( which you've already said she cant ) and also come to an agreement with the ex to buy out his share. Has your sister in law spoken to her ex partner and discussed this yet.
I had the same problem with my ex when we split, I moved out of the house because i couldn't afford the repayments by myself and he's stayed in the house, but i dont pay anything towards the mortgage why should i, i dont live in the house i have my own rent to pay on another house.
As it turns out he's having problems paying the mortgage anyway and were going to rent out the house and he's moving back in with his parents.
Especially with the way the market is now it's going to be hard whatever you decide to do.
Now in saying all this me and my partner wern't married, ( since she's your sister in law, i take it that she is,unless i've figured this out all wrong lol ) then i would of thought everything changes i dont know as i was never married. The best bet would be to get in contact with a solicitor or go to the citezens advice they would have much more upto date information than what i do.