Because if I were in the same position I would finally have the chance to even up the financial support for all my children
This is where you are wrong, in my view. It's not MIL who has given money unequally as she didn't give her children any money. Her father (your DH's grandfather) did that. So the unfairness stems from grandfather- why resent MIL over it? Basically, it is coming across to me as though you feel entitled to a share of her inheritance to even out decisions her father made. That's not really fair.
Why should she be giving you her inheritance, which she may have plans for and need that money to support her financially in retirement? Or perhaps it will just make her life a little more comfortable as she ages. Obviously, I don't know her financial situation, and doubt you do entirely, so unless she is very comfortably off it may be possible. Either way I don't think it is yours/DH's by right or even morally. Your financial situation is irrelevant, TBH. You chose the lifestyle, including number of children and have to cut your cloth according to your means. It's nice for parents/families to help but you shouldn't make financial decisions based on assumptions about borrowing/being gifted money by then, I'm afraid. My DB does this and it drives me mad.
I don't know where she would stand as an executor of the will as regards to demanding repayment of loans her father made to your DH's siblings- I imagine it very much depends on whether there was some legal agreement made between GF and DH's siblings. If there is no written/legal agreement then I should think it would be a lot more difficult to prove it was a lone as opposed to a gift. That would mean MIL taking legal action against her own children- not very likely in these circumstances I would have thought. Especially if you are right about one of her son's being an aggressive bully....
Finally, have a little bit of compassion for your MIL. Her father has died. She is having to deal with the well and his estate, that is stressful. I'm not clear if the £2000 to each relative was stipulated in the will or whether MIL decided to gift each person that. If the latter, she has already been generous.
However, I can understand feeling you'd like more help with child care- although as you can work your shifts around childcare, strictly speaking you don't need it. But it would be nice to get some more help, I can understand that. The difficulty for parents (quite often mothers) who have more than one child is trying to be fair when it comes to offering childcare- and that's not always the same as equal. And she does need some time for herself too. If you really want more help, perhaps you need to ask rather than just waiting to be offered? Sometimes if it looks like people are sorted, it doesn't occur to others to offer help where it doesn't seem to be needed/wanted.