It's an incredibly difficult situation to be in.
My brothers 2nd wife is basically in your position the only difference being that brother and 1st wife were already divorced when she died.
Grief can hit people in odd ways. My late sil family when new sil came along also struggled and didn't feel able to meet her straight away.
When they did, I have to say they went out for a meal so they were on neutral ground? It also meant there was something of a time limit as you don't stay for hours in a restaurant after you've eaten as you might at home.
I think that did make it easier for everyone. Especially if you are living in the deceased mum's former home?
I think in your shoes I would say to dh that you think it would be easier for all concerned if a neutral spot for a meal were chosen.
I know they're an mn hated thing but possibly a "family" pub with a play area attached if dc are going to be there (do they have dc too? Is this because they've recently become parents themselves?)
That would allow for the odd moment at least for just the adults to talk or even that first meeting could be adult only an evening meal?
I think baby steps would be good here.
I'm afraid I do think that while I understand their raw grief at the beginning/early stages their behaviour since has been poor and not prioritising your dsds needs above their sensitivities.
Another relative of mine was "widowed" young (they weren't married living together) and her "in laws" behaved appallingly - they basically booted out of her family home and they never had contact with her or the dc after the father died awful situation!
People can be very strange at times
And I think you all (you, dh and dsd if she is involved/knows) need to be prepared for them potentially backing out again too
Dh doesn't seem to have put much thought into this at all has he?