OP, how recently (or otherwise) was it that your MIL made the infamous comment about wishing your DH and his ex had never split up? If it was in the early stages of your relationship then, hurtful as it must have been to hear at the time, perhaps it's time to let that go and move on from it secure in the knowledge you've proved your staying power. She may well have changed her mind by now anyway if she has seen how happy you have made her son. If it was within the last few weeks or months, then YADNBU and your MIL is an insensitive weirdo!
I totally get why you would feel uncomfortable about this (I'm a second wife myself, although my PILs are no longer with us). I think some of the comments are harsh and unjustified, as if you're demanding your PILS should drop ex-DIL altogether which you've quite clearly never suggested. I can also understand how you can get to a point after a long period of time where you think "jeez, can we not have ONE family event without DH's ex?!" which can come as a surprise to those who think you've been fine with it all along, but it's like drops of water wearing away at a stone, especially if at every event for 15 years you've spent it in a state of "forced politeness" and never felt truly able to relax.
It's all very well to praise the PILs for having this mature friendship with their ex-DIL, but unless they originally asked their son how he felt about them inviting his ex to the first post-split event and he made a classic don't-make-a-fuss response of "it's fine", which they've taken at face value ever since, then it's a little insensitive of them to not at least consider how her continued presence might make him - the reason she's part of their family in the first place, and their own son - feel, never mind the OP. No question that she should be involved in any event focused on the children, but it is possible for the PILs to be a little more selective otherwise: invite ex-DIL to enough occasions to reassure her of her continued importance to them, while maintaining their friendship privately too, but having the odd smaller get-together without her.
Of course, they could be sitting there saying "oh well, I suppose we'd better invite DS's ex again, it would seem odd not to after all this time, honestly I thought she'd have found herself someone else by now and be too busy to turn up here all the time"! Whether there's any basis in truth for that or not OP, maybe you could tell yourself that's how it is and so help yourself feel less uncomfortable at the family events 