"My MIL told me the argument that I was no longer invited to the reunion but my husband and sons were still invited but I wasn't???? "
What was her expectation of how this would work? Were you supposed to still be there, but hiding in your room wearing sackcloth and ashes? Or did she envisage you skulking home with your tail between your legs, but somehow leaving the car there for your husband to get himself and the boys home? How was that supposed to work?
"I told her that if I was not going then neither were my boys, my MIL got quite pissy saying that it would be unfair to my nephews as they were looking forward to seeing my sons."
Strikes me as desperate scrabbling on her part, that her having a go at you would have Consequences Visible To Other Family Members. Now she'll have to explain to them why you and the boys have left. That'll be fun.
"My DH went out quite early to meet some of his childhood friends as he grew up here and because of that he missed our argument."
How - convenient for him. After all, there was no way his mum wasn't going to notice, so he made sure to absent himself and leave you in her firing line. Cowardly, and cruel. He'd be getting both barrels from me for that alone.
"I've just got home with my boys and I've just received a text from my dh telling me that his mum explained the whole situation to him and that me not letting the boys go was very immature."
My only response would be 'And you're sure your mother gave you an unbiased explanation to cover how she managed to drive your wife away?'
"I have to pick him up on Monday and apparently, my mil is very upset about her grandsons not being there and wants to talk to me before we leave."
He can get public transport, I would NOT be picking him up. Or, the mummy's boy can stay with his mummy. And - she "wants to talk to me before we leave"? Presumably to give you another scolding? Well, the pair of them could fuck right off if they expected me to stand there and take that.
"My mil has never liked me, she straight up refused to come to me and my husbands wedding ..."
I'd be reminding him of that.
It's high time your husband prioritised his wife being upset by his mother, over his mother upsetting herself.