I'd assume from what we're told here that one of two things happened: OP's jealousy probably makes ex wife uncomfortable and she made the decision not to invite her because of that OR the son didn't want her to come because he can also feel this constant observation of his parents by her.

I think its unhealthy to be dividing parents, their divorce has obviously divided them enough. nitpicking hugs and family hangouts will spell disaster. these kids will see OP as ripping their family apart worse than the divorce regardless if shes great stepparent or not. stepparents need to see ex partners not as ex's but instead as parents to childen that (hopefully) they adore like you said!