DB and DSIL are in the process of separating.
He has left the family home, they have 1 child aged 3.
They are married and both names on the mortgage.
They also have additional debts.
He is intending to pay (far more) than required CSA payments, keep lots of their joint debts but ideally would like the house to be sold and then split assets.
DSIL is being given conflicting advice from everyone and is now under the impression that she can remain in the marital home until their child is 18 with DB continuing to cover his half of mortgage payments.
I have advised her to seek legal advice but it's so soon after their split that she doesn't know her arse from her elbow 
It's tough because I love them both and want to support them both, but there is no way he can sustain the necessary amounts and be able to move from our parents spare room before he's about 75!
So main question- does he have to keep paying the mortgage, or could he eventually force the sale?
Apologies for the rambling incoherence 