MIL and her priorities!

But let's not forget he is grieving too.
If you are happy for the MIL to behave in the way she is, then the OPs DH is entitled to grief behaviour.
Yes, it sounds as if he is being petty, but he is grieving. He's also watched a parent, his dad, decline cognitively.
Some very intolerant and misinformed here today

Get your point and I recently lost my Mum so I do get it.

But the way the OP has written it her DH sounds very judgemental to me and thinks his Mum as just his Mum not a person in her own right- which is odd for a grown man.

Can’t you take your baby to visit her? Keep your connection going?

Whilst I agree that mil has a right to this, I wouldn't blame her dh for his response.
Just as his mum has lost her husband and is doing slightly irrational things, he has lost his father, and to some extent the mother he has always known. He is also allowed to overreact a little bit. It's never nice to see your father replaced so easily so to say. He can also grieve the family dynamic that he knew. If he needs six months to come to terms with his mum's new found vibrance in life, that's also fair.
(Sorry not picking on you, just replied to your post, many was saying similar to you)

I think the ‘hasn’t bothered to pick up the ashes.’ comment is very hurtful, I sobbed just picking up my dogs ashes!
My mil had dementia and dh always said he lost his mum long before she died. Your mil lost her dh long before his death.
Personally I think there’s a chance your mil is going through a sort of hay day period and it may not last.
When my df left my dm she went ‘off the rails’ for about 2 years before remembering she had a family who wanted to see her.
Try and be neutral about your mil as if she wants to come back to being a doting gm again you will need to help facilitate this.

I think it’s unrealistic of you to expect her to be coming over whilst you’re DH is barely speaking to her. She would feel pretty awkward about it

Let her live her life ffs.

Because she has been through a major life change?

The sad fact of the matter is that people can find a new spouse or partner after their first has died, as your MIL is now rapidly showing you, but no one can find a new parent. MIL is not the only person grieving here. Your DH’s world has been rocked on its axis and won’t ever be the same again. He has my sympathies. 💐

This is a very uninformed', one sided response.
If you can see mil side, you should be able to see dh's too as they're both grieving.
Mil is doing the classic let your hair down thing, which, unlike you think isn't her just living her life and making choices. It's a textbook, ' I feel lost, I've lost my lifelong companion and I don't know what to do. I'm hoping someone new might fill that void'. It really is almost a cry for help. Like a teenager behaving poorly because they're so insecure and don't know what to do.
For example in a clear moment if you were to ask mil whether she wanted a relationship with her grandchildren for years to come she'd almost certainly say yes. Similarly she probably doesn't want to hurt her own children. Yet, she's unable to reconcile the two things at the moment.
DH is grieving a father and the mother he knew. It is hard watching your parent go through the classic post spouse's death script.
Cut him as much slack as you're doing the mil.

It's messy and your DH is hurting right now. That's ok. But it's not acceptable for him to "hit the roof" with his mum who has had to see her husband fade away.

If your DH wanted her to feel welcome and involved in your family he should have tried to see it from her side.

Maybe she did but he's pushed her away by making clear he disagrees with her choices.

Absolutely fine. He's hurting. She's hurting. Hopefully with time it will heal. Right now your child isn't the centre of her life and that's fine.

Under his disapproval. And your disapproval as well otherwise why are you on here? Who needs that shit

Personally I’d give it a rest, let her get on with things and keep the door open.

That’s ridiculous, OP. Why would she assume her DS’s wife wanted to see her if her own son won’t speak to her? Or think sneaking into the house while he’s at work is ok?

And people do all kinds of things about ashes and graves. My mother was horrified that it took her friend’s DH so long to put a headstone on his wife’s grave, but he just couldn’t bear thinking about it.

Your MIL has retreated into a bubble a fantasy to give her poor damaged heart and psyche respite from the hell of the last few years. On some level her lover is making her feel literally care free and she can't think of anything or anyone else just now
It seems hard that this selfless womans current behaviour is negating all the good that came before. Bereavement is a bomb in a family tho. Certainly in our family when our mother was widowed she behaved awfully to us her children. She only saw the loss of our dad as hers alone. She was nasty and dismissive and angry. It took years for everything to right itself and for the family pieces to reassemble into something different with dad no longer there. Some of my siblings never forgave her and are now estranged. For those of us that battled through we have a new relationship with her and are damn glad we hung on now. The thought of her dying and not being friends with her would devastate me. Grief is hard enough without those complexities.
It's likely your MIL will emerge from this fantasy and the real hard work of grief will kick in. She has lost her person and can't outrun the grief forever
Your DH has lost his dad and his mum is currently unreachable. How very sad and distressing for him.
I would be encouraging him to have some support through this. Therapy helped me to manage my own feelings during these years. It helped me to acknowledge my own loss and find ways to connect with my mother when her grief made it so difficult
As for yourself. Try to put your wishes for the grandparenting you want for your child to one side for now. There's time to fix that later. I would be encouraging my husband to see his mums current behaviour as a distressed woman in crisis who needs compassion. I would be reaching out with gentle kind texts to her. Suggesting meet ups that don't make her feel like anyone is looking for more care for her to give.
Good luck. This is hard.

Ah ok I see.

Has he spoken to her much since the new partners? Are you relaying his feelings to her?

Could you try just inviting her round for dinner?

I think you and DH need to massively dial back your judgements and expectations.

Caring for someone with dementia is one of the hardest things you can do - and if MIL took solace in the arms of another man to make it that tiny bit more bearable then, well, good for her.

Neither of you seem to understand that your MIL has been grieving her husband for probably years before his death - she lost the man she was married to a long, long time ago and had to come to terms with the fact that he was "gone" long before he actually went.

Good for her I say!