I'm amazed at the amount of posters describing the MILs comments as "mild." Mild, they're not! They're extremely rude and undermining. I've suffered my fair share of rude family members, so I get it. Maybe it's that much more irritating BECAUSE they're people you can't easily get away from. Perhaps people who haven't experienced rude family members can't really understand. On a different but related note, I've also noticed that some people are appalled at family estrangements no matter how abusive someone has been. I think there are people who just have no idea what it's like to have a toxic family member and can't relate to it. Also, some people can't relate to anything unless the same thing has also happened to them.
I heard a funny line once. "Ahhh, family. The other F word!" 😂
Can you take your children to visit MIL along with your DH? I think now is an excellent time to take some photos of them all together, and get someone to take some including you too. I also think you should take some video. I think your DH will find photos and videos of his mother and children together comforting in the future.
Your DH is just realising that his children will never know his mother, and his heart is breaking. 😢
I was lucky enough to know all four of my grandparents well, and the nice thing for my parents when they got elderly was that I could freely discuss their parents with them and share memories, and say things to my parents like "You sound just like your mother!" and "You're looking so much like your dad these days." It's really lovely when your children know your parents and it provides a wonderful generational continuity that's very comforting. As in, when you're old, your parents are long gone, but your children knew them.
That's what your DH won't have, and that's super-sad for him. And no siblings either. Poor guy!
You haven't experienced the death of your mother yet, but let me tell you, it's a pain that's totally unimaginable until it happens. The whole world seems grey without them. It's as if the whole world has changed. It's a very, very strange feeling when they first go. It's like, "Where'd they go? They can't just have disappeared." It's extremely freaky. I thought I was prepared when my mother died after a long illness, but after she actually passed, I was on a different planet, one I couldn't have imagined seeing beforehand.
After the death, I was on a bus going to the hospital to do some admin, and I saw this old lady, about 85-plus. My mum was 70 when she passed. I looked at this old lady and thought, so wistfully, "Oh, Mum will never be that old." And about six weeks after, a stranger snapped at me, and I cannot tell you how much it hurt. At that stage, you are a giant walking bruise.
Things do normalise, but it takes a long time. Your husband is going to need you badly in the times to come. In my experience, parental death puts a huge strain on a marriage. But like everything, the season passes eventually.
But be prepared for thing to be very difficult for some months. You will probably need to take regular breaks by seeing friends, exercising if you can. And don't forget, however hard it is for you to bear witness to your husband's terrible grief, you will go through the same one day and will need him.
Things you can do:
Take the kids to visit MIL with DH.
Take photos and videos of you all together.
Research online ways to support someone who is grieving
If MIL has cancer, make use of MacMillan support services for family members. They are brilliant.
Wishing you the best of luck.