We were in the same position with my parents a few years ago. Dad with progressively worsening dementia, Mum pretending there wasn't a problem. I live at least 90 minutes drive away, up to 2.5 hours if the traffic is bad. I have three older teenage children, I could not be a "carer".
Few points - they might not have a social worker. Having a dementia diagnosis does not mean you automatically get allocated one. Dad's dementia was managed through the GP and mum had a "mental health" number she could call. They never ever had contact with social services at all. I know this for a fact as as we were reaching crisis point I called social services to ask what would happen to dad should mum be ill as he would not be safe on his own. They reassured me, but also sounded concerned that they were not on the social services radar at all.
A lot of people confuse "care" with "help". Of course most of us with elderly parents are happy to organise a tesco delivery, do some gardening, make sure the bills are paid. Caring is a WHOLE other ball game, you effectively have to put your entire life on hold to be there 24/7. It is a massive ask and in the OP's position where they are 4 hours away it's just not practical in any sense of the word.
Accepting carers is very difficult - some people with dementia find any break from the routine impossibly distressing, new people in and out of the house very confusing and upsetting. And you can explain it to them for the morning visit and they've forgotten an hour later. Imagine the fear/upset of thinking you have intruders in your house twice a day. It probably will have to get to some sort of crisis point where something happens and your MIL realises the situation is just not sustainable.
Wishing you strength though, it's not easy.