from the understanding i have, jealousy is more tied to a fear of losing something you have to a third party, whether it be a relationship or so forth (it also usually contains a bit of resentment/hostility). it works here, as it xiaoyu is clearly bothered by xinrens return and her sudden reappearance and casual closeness to ye, as if shes going to lose her.
being envious on the otherhand would be a bit of admiration and discontention, the want of something someone else has, but it doesnt need to contain hostility, just the want. it doesn't really explain the full scale of emotions xiaoyu feels when she thinks of xinren.
due to their overlap though, it is possible for the lines to blur or for someone to experience both. the comment about wanting to have been about the same age and meeting ye earlier is a good example. envy of xinrens shared childhood and experiences with ye, but jealously on the sense that if they did meet earlier they could have formed a stronger relationship and have more expriences together while xinren was still out of yes life.