I have been thinking about this in terms of myself
It is really hard to do this without the actual amounts to show where she loses and he gains but I will try. Also basing it on where I live in the SE of England
Say OP earns £15k and takes home £1000pm, then gets £1000 in benefits totalling £2k and that is what it costs pretty much to run her house and family she is left with nothing over.
DP earns £45k and takes home about £3000pm, a single man in a house with a mortgage I could only estimate his outgoings at about £1500pm
OP has no deposit? And low income So don’t know how they would buy a house
So they rent a house for £1500pm and DP rents out his owned properly, covering his mortgage and keeping his investment safe.
So the total outgoings with rent is approx £2500 pm for all 5 of them, with rent and bills
His contribution to the rent and bills is not 50%. He wants to pay 25%. So he gives OP £650 odd per month for his contribution
OP loses £1000pm in benefits and earns £1000pm
She cannot physically make up the shortfall of £1850 per month
To compensate her benefits, he would need to pay £1650 towards the house and bills which is more than 50%. She would still have virtually nothing left after contributing as she no longer has the benefits.
He may possibly be slightly worse off, but it is similar to what he was already paying for himself. He still gets £1500 or so to himself per month. OP has no spare money after bills
This is why it won’t work
Not because he is horrible
But you do not have enough to bring into the RS to make it equal at all, and you will resent each other. Whether it is right or wrong for him to pay less than 50% it just isn’t physically possible. And if he compensates you for your loss in benefits this is the same thing - he’s still going to be paying more than you do