OP, I take it you still have your parents, as otherwise you wouldn't be behaving like this, I hope! The fact is, that no one knows what it's like to lose a parent until it happens, you may think you know, but you really have literally NO IDEA! Your poor DH has lost the man who brought him up, and has realised that life is no longer certain. In his mind he may be terrified that his Mum could go at any minute, and therefore he wants to do what he can for her.
Have you actually talked to him about this? If not, then please do, but be gentle, parents can turn into God's when they're gone, and the last thing you want is a row. So I suggest, you explain to your DH, that having to cook and 'entertain' his DM every Sunday, is getting a bit much, so would he mind terribly if once you've done dinner, you go out with a friend, go upstairs and read, or watch TV, whatever you would prefer to do instead of talking to his DM. Perhaps you could suggest that once lunch is over, you all go for a walk, or DH takes his DM for a ride in the car, or to a garden centre, or somewhere else that she may enjoy.
As a matter of interest, are you close to your parents, both emotionally, and physically, as you really don't appear to be showing any empathy whatsoever?