It's not very common I don't think, and contrary to the reporting of the case in some media, I don't think it will massively set a precedent for future cases. This case was unusual in that there was explicit, written evidence that the mother and siblings wanted to reminburse the lady for the care she was providing. I think it's more common for a sibling who's providing care to be left a greater share in the will, and perhaps claim attendance allowance or carers allowance to help with day to day costs, although obviously both those solutions come with problems too...
That being said, if you read the full judgement on Bailii which explains how the judge reached his conclusion on how much was reasonable, and you see the detail of how much this poor lady needed in terms of care, the 'deal' the siblings could have reached of their own accord to pay their sister something in the region of £150,000 (which she broadly calculated as £135 per day for 900 days of care plus some incidentals), for which the mother got room, board and 24/7 care from clearly very dedicated relatives, looks pretty sensible/reasonable. You couldn't source a full time professional live-in carer for less than that (plus the mother would have had to buy her own food and drink, pay bills etc on top) and tbh it sounds like for the last year of her life at least she would have been beyond what one carer could cope with and would have needed a care home which is much more expensive - £1k per week plus. The mother very easily could have racked up 200k + in care bills over that 2.5 years. So looking at it that way £150k for 24/7 highly dedicated care is a bargain.
But these things are really hard to calculate reasonableness of as it's so individual to different circumstances, which is really why this family and others in their position should have had these conversations upfront and documented them and their rationale - as you say while it's not inherently unreasonable to pay out quite substantial sums for care, where that's going to a family member HMRC may be interested in whether these were gifts or genuine payments for services. There's the fact the mother moved into the daughter's home, which on the one hand is obviously a big commitment but on the other hand more convenient for the daughter, and how much is reasonable for the mother to pay for room/board/household contribution. Then there's the fact that at first anyway the care wasn't a full time job, the daughter and her DH were certainly limited in their opportunities for other work and to enjoy their retirements and hobbies etc but they weren't literally caring for her 24/7. And then you get into how much the daughter would have been doing for her mum anyway even if not her official/ paid carer, and how much the other siblings would have stepped up and offered a cheaper alternative, the eldest brother gets very short shrift from the judge and didn't even know what health conditions his mum had, but several of the other siblings clearly were close and were much more involved, one lived very close and might have supported if she'd lived at home with carers, one offered to have the mum live with her for £100 p/week, now who knows, quite possibly those plans were totally unrealistic and would never have transpired, who knows. But the point is it's not easy retrospectively to try and decide.so not advisable to do so really!