Wow. This is a mess of things going on, isn't it? I have so many questions raised by your first post!
"My DH booked a big holiday for this year for me and our 2 DC."
Am I reading the implication right, that this is a special holiday, out of the ordinary for you? And also, why do you express it as being for you and the DC, not 'us and our DC'?
"Then my mum also booked to go to the same place along with my sister and brother - without telling me first."
Why do you think she didn't tell you? Oh, because she knew this was not what your husband wanted, didn't she? But she thought she'd get her own way if she handed you a fait accompli. Has she done this before? Sounds like she might have done.
"My mum's logic was that she would be able to help us out with childcare and at the same time we could get the big family holiday she's always dreamed of since my Dad passed away in a horrific incident many years ago. Not the greatest logic but her heart was in the right place."
So what she wants overrules everyone else? Like your DH? And you? And your dad passed away 'many years ago' - how long has she been harbouring this dream? And how often is the 'horrific incident' aired as a sort of trump card to get her own way? (I'll come back to guilting people later.)
"My DH kicked off big time and changed the dates of our holiday even though it was unlikely, due to the nature of the type of holiday it was, that we would spend loads of time together anyway."
Him kicking off was not a surprise. It's a pretty natural response to being railroaded, and having your holiday hijacked definitely counts as railroading. How much time you'd have spent in their company is irrelevant, they're there.
"He paid for it as I'm not earning much at the moment so he saw it as his right to change the dates."
There's a possible can of worms there that I'll leave for the moment, but it bespeaks attitudes you and he both hold.
"My mum was gutted and crying that she'd caused this issue."
Or was she 'frustrated and angry crying' at not getting her own way? And yes, she did indeed cause this issue with her underhanded attempt at hijacking.
"I was really upset with him that he couldn't just go with the flow and let us have this holiday together."
So you were fine with the hijack?
"I understand his point of view but my family aren't monsters and we all get on well."
They don't have to be monsters to not want them on your holiday. Your hyperbole is interesting.
"My mum literally does everything for us, she babysits at the drop of a hat, dotes on our children, is always there for me and needs as much family support as she can get as she's still grieving."
Aaaand, we're back to the guilting of people!
"I now feel that if anything was to happen to my mum in the coming years and I missed out on this last big family holiday together I don't think I will ever be able to forgive him. AIBU???"
Guilt, guilt, guilt (sang to the tune of Spam). Guilt that you're feeling and guilt that you're trying to lay on your husband.
So my BIG question is, is guilt like a currency in your family? Is it how people are controlled and manipulated?
And just in case you're in any doubt, YABU with your 'can't forgive' grandstanding.