"He is also a co parent with someone else and it's good that he supports his co parent with parenting things"
What a load of sanctimonious rubbish. I don't know whether to laugh or cry.
He is ALSO a co parent with someone else who isn't the ex - i.e. the OP - and it would be good if he supported his other co parent with parenting things.
Except, it seems, he often fails to do so.
This man has three children, not two and he has an equal responsibility towards all of them. Similarly, he has an equal responsibility towards both the mothers of all his children. It certainly shouldn't be the case that the one who shouts loudest or threatens to get "upset" has him asking "how high" when she demands that he jumps.
This man also chose to have another child. He should only have done so if he was prepared to commit himself to them all. And while he can't be in two places at the same time, nor please everybody of all the time, if he was fair about this all, it should pretty much balance out in the end with all the children sharing his time.
I bloody hate this whole narrow minded attitude of, effectively, "they were here first, they're more important, they get prioritised" and so on. Funny how people with that sort of attitude towards children from the "first" family never seem to apply the same sort of questionnable and unpleasant logic towards the children (plural) from that "first" family isn't it ? No-one who sounds off about "second" kids and "second" families and how they should suck up what boils down to a lack of respect, a lack of loyalty & commitment, hurt, and favouritism etc ever suggests a distinction between the "first" kids themselves .... so, the 1st born is entitled to more of dad's time and attention than the 2nd born and so on.
Anyway OP YADNBU ...... you asked him with plenty of notice and doublechecked he was able to do this. It wasn't up to you to specifically make a point of asking about the usual hockey .... after all, such a "committed" father surely wouldn't have forgotten would he, and, as a grown man, I don't think it was unreasonable of you to assume that he'd made other arrangements to cover that activity off. But whatever, when he "remembered" he should have been grown up enough to apologise about it and work out a solution - did he even try negotiating with his ex for example ? And if not that, heavens how difficult would it be to take along a baby in a pushchair ? I'm not sure if he simply can't be arsed, or he did indeed ask the older kids if they minded ? I suppose he could just be saying that as an excuse but if he really did ask them I think that's disgusting. He's supposed to be the parent - he's supposed to make the decisions. Allowing them the choice of whether or not he brings their sister along is divisive and gives them an inappropriate level of power about something they shouldn't have a say in. It's hardly as if she's going to affect what they're doing after all ..... and the very last thing he should be doing is reinforcing his own attitude about who takes priority by getting the kids involved. Does he really want them growing up thinking they're the "most important" kids because they'll undoubtedly play on that as most kids would.
OP .... I really feel for you. He's done a rotten number on you and he owes you one to make up for this. But he also needs to understand that he can't carry on like this as favouring his ex and/or the older kids will destroy your relationship and could potentially harm your DD when she's old enough to realise she's second best.