I could literally weep for you, your husband and your son-in-law, but that will not help you in the slightest.
You are in a very bad place, right now, and I do understand why you feel your son-in-law is being disrespectful BUT
we all deal with grief in our own way and are you absolutely sure that your daughter did not, I am assuming she died of an illness and knew she was dying, forgive me if the assumption is wrong and she died in an accident, tell her husband that she hoped he would find someone else to love after her death?
If you possibly can, let your son-in-law come and explain and try not to make it too obvious that you find this development too soon and disrespectful to your dear daughter and to your own feelings and grief.
It is very hard to be a widower with young children, even when, as your son-in-law has, there are parents-in-law running themselves ragged to help.
If at all possible, breath a sigh of relief that this unhappy man has found a glimmer of light at the end of the very dark tunnel you are all walking through.
I am sure the only light you can dimly see is that coming from your grandchildren, so please don't do or say anything now that might, just might, make seeing them in future hard.
And off-load on us on here, and to a good friend, ær a priest or minister or a grief counseller.