I understand your frustration where you see unfairness in the systems, however if you really want to make some change, rather than complain on here, why don’t you lobby your MP!
Even if you were to stop paying anything out of your income tax for people of working age (non disabled) and their children, the amount you’d stop paying is negligible, maybe 3-4%. If it were reduced only for those that may take advantage of the systems, then it would be far less given it amounts to less than 1% of government spending overall.
So, based on that logic, it just seems like the issue isn’t the money as such. Are you equally cross about spending money more widely on benefits, so disabled adults, or maybe pensioners? Maybe foreign aid? Otherwise it doesn’t seem worth your ire to be so clearly disgruntled by the real impact on you, which as it seems is financially negligible.
Lastly, you know nothing about this family from what it seems. Maybe her disabled child doesn’t like the husband, maybe change pushes him off base, maybe, perhaps mum would love the husband to move in and give her some support. Maybe the mum is up all night with her disabled child!
Maybe have a touch of compassion for a situation you know nothing about.