You admit yourself you don't know the facts of what's occurred between your DM, b and sil and their children, so you'd be wise to try and remain relatively neutral.
I do think it was unreasonable to totally make all the decisions regarding your DMs ongoing care without even discussing with them.
That's just further proof that they're not afforded the same consideration and respect as you and your family are.
Also honestly if your DM is at a point of needing 24/7 care it was a foolish move on your part as you will need a break at some point and if you piss off b & sil this early on they're not exactly going to feel particularly open to providing respite care are they?
"And if DB made more of an effort to visit with his DC, the emotional bond would be stronger" the emotional bond between mother and child is first developed in childhood when the parent has the power and responsibility to start that. Your DM hasn't always been older, unwell and unable to visit your brother. Effort with an emotional bond works both ways, if she didn't visit them while she was still able, rejected Christmas invitations etc it's as much if not more her to blame as your brother.
There's only so much rejection or criticism that anyone can take before they naturally protect themselves by limiting contact with the person doing the rejection/criticism. We see it all the time on mn and posters repeatedly and constantly subjected to such verbal/emotional abuse are advised to limit/cut contact precisely to protect their and their family's mental health.
"Sometimes family pushes you too far." Exactly
"and has been in no position to help with childcare for many years." By that do you mean she helped you with childcare in the past?
And your brother having moved away is not a good enough reason to exclude him anyway, lots of adult children move away from "home" for damn good reasons especially these days. They go where their work is, or where their significant other is from or where there's better schools/childcare/opportunities for children, to access affordable housing or even to get away from an unhealthy even toxic family of origin.
With all the methods there are for staying in touch now too you & your mother really have no excuse why you couldn't have kept him informed.
It seems you both expect him to do all the running purely because he chose to move away.
"If anyone on here thinks that someone who is landed with the care of elderly parents is “the golden child”, then they’re seriously deluded." Actually as someone not only from a toxic family but who also worked in elderly care for many years that's exactly what I've seen happen.
"DB moved to be with SiL straight from uni which in itself speaks volumes" wow! In what way exactly? This is a very normal thing to do and in no way excuses his being poorly treated by you or your DM.
"To add DB would never acknowledge that's where the cracks stem from" are you blaming db for this?!
"I think your dm sounds very manipulative and is doing her best to drive a wedge between you and your dB and you are going along with it because you don’t want to rock the boat." Totally agree
"so no clue where that came from" actually it came from comments you made!
"How DM didn't ever want to spend Xmas with them (just to add they never wanted to come here)."
"Some of it may be true"
"I can't answer all the questions on whom visited whom and when."
"Never any invite for us to spend Xmas at theirs." I think you need to be prepared to possibly discover that invitations were extended via your mother and she rejected them and didn't tell you.
"Nothing wrong with moving Away from home - I did it. Sadly at the same time DB also cut all ties with his existing friends" completely irrelevant and actually none of your business!
I don't know where people are assuming it's DEFINITELY about money when he didn't even mention that.
For now - you butt out!
You admit you don't actually know what's gone on in their relationship AND YET you clearly take your mothers side!
You're hostile towards your brother and that will not help anything.
Stay out of it! And that includes not indulging your mother slating your brother.