I think you need to consider the visa criteria which is tough, the fact you will have to pay for all health care (which i suspect will be much more costly in the uk) and even then it will fall to you not your husband to look after him day to day.
Say he has a stroke at home and needs urgent medical care, you will need to call an ambulance, he will need a lengthy hospital and tests and then you will be his sole carer unless you fund him in a home. All of this you will have to pay for. That could be your life savings and retirement funds. That might push the financial responsibilities for you and your father in law onto your kids which they otherwise wouldn't have.
You probably won't be able to afford a home if things get really tough and if your husband is the main earner, you will have to do it cos you will be paying the NHS fees. There is no respite from it once you take it on. If financially its crippling you and he gets ill and you have the choice of phoning for medical care or not what are you going to do? The dilemma could be dreadful.
If he lives til his late 80s or more how old will you be? As his carer. And what happens if you are ill and cannot care for him? Again it could fall to your children. You aren't just taking the decision to take him on as a couple you are potentially making the decision for your children to take him on. Would you want this for yourself? Would you want this for your kids?
Care which could end up being 24 hours a day. So no holidays. No opportunity for you and your DH to go out together. When your kids get older, and potentially more away, no chance to go visit them.
Your husband makes out that its simply a case of converting part of the house. Its really not. And as difficult as it is, it would be better to be arranging appropriate care where he is simply because the burden and responsibility wont destroy your life.
The poster above saying that if you don't take him on it will end your marriage isnt accounting for your life or marriage ending because you took him on. Realistically there are scenarios here where you and your kids would be better off if you walked away and left your DH to deal with it all alone if he moved here.
Honestly i think you would regret and resent taking him on. Your husband is minimising the whole thing massively.
For you i dont think there is any choice but to refuse unless you are prepared to potentially throw away your life and commit to being a carer. Is that what you want? Is that what you want your marriage to be? Is that what you want your family life to be?
I'm not sure i buy into the idea that there are no alternatives where your FIL is. They might be difficult to source and arrange and may cost money but that may still be cheaper than paying for health care here, which he won't be eligible for. I suspect your husband is looking for a solution which is easier for him to sort out. One thats not necessarily the best one for anyone.
Anything less than a firm no, you will live to regret I'm sure.