How to Deal with Jealousy and Dynamics in Extended Family

I would tell your sister (for example), "Remember when we were all kids and Mom would take one kid with her and how you got some quality time that way? It's kind of like that. Sometimes the whole family together is overwhelming. It's not that I don't love all of you. But sometimes an outing is best for a subset of people." I think the newness will wear off eventually and things will settle down. You might also try talking to your mom and asking her why the parade is so important to her and maybe getting a little better understanding of each other's POV.

Some of my family cannot fathom not being included in everything. But I'm OK with my sister having Thanksgiving with her ILs. Why shouldn't she? They're her family, too.

And you can say that, "You know, when you and SIL go out and don't include me, I know that's not personal, either. So please don't take it personally the other way around."

I think the newness of it all will wear off soon.
I think that for things like shopping, it's fine to want to go with just one other person, no big deal. But, for something like the parade I do think you should invite your mother...especially since you just got back to town. Only because a parade is something that everyone in town usually attends, I think it would be kind of strange for your mom to not go with her grand kids and have to go alone or something, sorry. Grandparents live for that kind of stuff...my parents drove 45 minutes just to go to the 4th of July parade with my kids. You don't have to invite her to everything, but some things you just do, it's not nice to exclude sometimes.

How were they before you moved to town? Did they ALL do everything together most of the time? If not, then my guess is it will die down once you've been there for a while and everyone gets used to not having to soak up every moment you're in town.

If they were generally all together before you moved back, then I think it's unrealistic for you to expect them to change.

Scenario 1 is more likely to me. It will get better with time.

I have the same problem with dinners, parties, etc. My mom and dad split before I was born and both have remarried (and since divorced again), dad has other kids but mom does not. I actually lived with my aunt and uncle so their daughters are like my sisters rather than my cousins. One still lives at home...same property but different home (although she's almost 50 and her son and his gf lives there to). When I want to invite just the "parents" it's hard to get the one cousin to not be included (along w/ the rest of the household) so then I feel weird that the other siblings (including her sister and all the nieces/nephews) are not invited. You can see where a meal with us and 8 "parents" turns into a zoo of 40 people!

The other problem is that sometimes my aunt will extend invitations to her other siblings...not all my aunts and uncles are invited she is because she's more like my mom and my dad is because he's my dad but that does NOT mean the whole "family forrest" is invited....just the immediate tree. My MIL does the same thing too.

For my son's birthday he wanted to go to dinner with us and maybe the grandparents (not all of them could attend) but was ok with cake at home being with everyone (again, not all could attend) so when we sent the invitation to just the grandparents it specified "grandparents" and then a separate one went to everyone regarding cake at home that said "we will be having an early dinner w/ grandparents and will be coming straight home to celebrate with everyone".

All I can say is Good Luck and tread lightly.

You need to practice empathy. Pretty soon you're going to be the mom that your kids don't want to invite. If you think you'll be fine with that, you are in denial. Take off the rose colored glasses and let yourself feel the hurt of exclusion by people you put your life aside to raise!