I can guarantee you that she paid for his ticket to Mexico. Also, they haven't skipped off into the sunset without a backward glance, have they? Whilst they were miles away from home, and meant to be having the best time, YOU were in their thoughts. Now, why is that? I would suggest that he either talks about you too much, or she is very insecure where you are concerned. Again, why would that be? Clearly she sees you as a threat, as does the other woman. That should really bolster your self esteem. They see you as a credible threat and a danger to their relationships. And in that case, their relationships must be pretty shit.
Op, what you see with your Ex, isn't what goes on behind closed doors. People rarely change. Add to that the stress of babies that don't sleep and all that goes with little kids, I know full well that the couple you see for a 5 minute snapshot will not be reality. Add to that, they will be eager to present themselves as perfect to you.
Me and DH bumped into a couple we hadn't socialised with for a few years. They looked really happy. Their social media is full of happy family posts. We got together a few weeks later, and at the end of the evening DH and I agreed that you could see their marriage was in real trouble, the vibes were bad. What you see in 5 mins, is not reality.
My first H treated me terribly, cheated lots of times. I left him (also after 20 years). He has cheated on every subsequent partner since. Leopards don't change their spots. Even if they try hard for a while, they will revert to type in the end.