Looking at the original post again, I can see that the GP might have decided to give equal amounts to the 2 sons' families. One son has 2 kids, but the other has 5. In their minds it might seem fair....and it's their money so it's their choice.
The problem comes when decisions are made way back, when financial circumstances and nos of children were different. They prob never expected their DS to have a second family, nor to have to give to 3 further kids. If they chose to give your family the same as the brothers, but to let you divide it 5 ways, I could understand that.
It is what my DHs maiden aunt is doing - she is dividing her wealth amongst her 3 nieces and nephews equally, with each third to then be shared amongst the niece/nephew and their children. So, the single niece will get a big pot of cash, whilst the nephew with 3 kids will see his family receive the same in total, but each person get 1/4 of the amount that the niece will get. I can see the reasoning....and it's her money, so she can do what she likes and we will be glad with anything we receive.
OP I can see it is hurtful if your kids are seen as less important because your DH has already had his share of the money and that has gone to his first 2 kids. Perhaps they judge his choice to have a second family and that added to their constrained circumstances have affected their choice. It's not kind and it's not inclusive, but it is their choice. You are not entitled to anything from them and your kids are not entitled to equal financial treatment.
I think your DH would do well to explain to them how you don't understand their thinking and finds it hurtful. I think understanding their reasoning, even if you don't agree would help you.
However, see the relationship with his parents and your children's GP as more important than the money......you'll need to work hard to overcome this, but you can do it and be gracious and you will gain as a family if you do, in ways more than financial....so be the big ones here.