I totally agree with the people pointing out that due legal process must be followed.
I’d also like to add my own and others’s anecdotal evidence on the issue of MIL’s expressed wishes in the last few years. Perhaps her daughters could or should have done more, but it is very common for unwell, unhappy, very elderly people to take things out a bit on those closest to them. My mum and so many others of friends I’ve confided in, would mournfully tell people asking in the village or friends that she didn’t know how I was as she never heard from me etc. Luckily I was reassured that they saw my car regularly parked in her driveway and one shared her experience of visiting her own mum to be told she didn’t see anyone from one week to the next only to share notes with her brother who was also regularly visiting… Similarly my daughter and I would call 5 times for every time she answered because she slept a lot and was increasingly deaf and couldn’t hear the phone over Radio 4 at top volume. When we did get her at least half the call would be her expressing astonishment at us having called, telling us she never expected it as we were busy etc. She remembered her mum getting offended at not being called… - and we’d be on the end of the phone thinking but I called you, - it’s always me calling you, why do we have to go through this every time …🫣😂
I was very fortunate and in my mum’s very last days she sort of acknowledged how awkward she’d been, astonishing really, and said something about being afraid she’d rather “spoilt things” 😢- of course I reassured her otherwise and talked about the lovely times we’d had. I’m sure my mum would have been horrified if someone had decided her entire estate should go to the RNLI or somewhere on the basis of moans made to a friend or relative, I wouldn’t put it past her to say something like that at her most miserable.
Not my mum, but I have also heard of people promising the same things to different people in the final years, perhaps in the sad belief that they need to “reward” them to keep their interest, or out of forgetfulness which leads to amusement after their death when every female relative plus the beloved cleaner think they are getting her diamond and emerald ring.
Basically I think you should proceed on a presumption of goodwill and the law, the law taking precedence. Maybe your DH’s sisters should have done more, maybe circumstances made it difficult for them, but I’d assume that your MIL at her best and well would not have wanted to punish them via her estate, and the fact that she didn’t take any action to make it happen would bolster that view. Your DH will inherit a third of the value of the estate and he can do what he wishes with that third, it is none of his business what his sisters do with their portion he sounds a bit Old Testament to me.