Purely on the niece issue... MOST teens start deleting as many family members as they can get away with off of their FB accounts. What was fun as preteens becomes either "uncool" or they want prying adult eyes out so they can goof with their friends/ try out inappropriate things and not get reprimanded for it.
A lot of what you are describing is just "family politics".
It's natural for sibs to talk / do a lot of planning / randomly get together / hash out their lives & or history... which creates 2 things:
1) Apparent "get togethers" that are just spur of the moment things / unplanned that others don't get invited to. One doesn't typically have to "work" with sibs ... the house is trashed, fine. There's no food in the fridge, no problem. The kids are climbing the walls, whatever. The moment friends and or inlaws get involved... the house needs to be cleaned (at least a little, while more often than not sibs will often cheerfully just step over piles of laundry and shift the newspapers to the floor), there need to be snacks in the fridge for other cousins, kids need to be in a decent behavioral place, etc. (My sis has come over with my son in a screaming temper tantrum and in his room... I would NEVER invite my SIL over when my son is having a meltdown, and if she just dropped by, I'd feel awkward.)
2) Premade plans that they invite others along on. I've called my brother up to ask about x... and 20 minutes later we have full on plans for y... completely unrelated to x (x being something like: What are you getting mom for xmas, or did you SEE the score???, or hey I have to take my car in for breaks where did you say you go for pads?)... and end up with camping plans in 2 weeks. At which point we start inviting others along on. AFTER we call our mum (my job to do that). Not for permission... but because she's the human date book in our family, and it's both expected and required.
Another aspect of "family politics" is the "Who goes first?". In most families there are the planners, the bringers, the talkers, the late ones, the idea people, the anal, the last to know, the first to remember, the reminder of others, the "don't EVER tell dad, until mom already knows" rule *everyone* knows, the tell AJ it starts 30 minutes before it does, or she'll be 45 minutes late instead of 15 minutes rule, the _______, the _______. Roles and rules, roles and rules, which are particular to each individual family. Over years and decades people fall into patterns that are almost IMPOSSIBLE to break. For example: Everyone calls my mum. She's the center part of the web for family plans. All invitations, ideas, etc get bounced off of her first... and then shoot out... web like to everyone else, regardless of who is actually planning the get together. In my husband's family, that role is held by my SIL. My SILs BILs who have married in, very quickly learn that any "all of us" thing goes through my mum... or it's not "real" to anyone. Just like I learned really quickly it's my SIL on my husband's side.
Family politics just get complicated. And when one marries into a family (in my experience) they do NOT change. My family has all their roles and rules and my husband and those who have married into my family only alter the body count. In my husband's family, I follow all of THEIR roles and rules, I haven't changed them a molecule. Talking with my sibs and their spouses, it's pretty universally true. Also, that anyone disparaging or trying to *alter* those roles and rules gets stonewalled. Not maliciously (at least not in my family), but "blank stare" like. Like someone is trying to walk around outside butt naked, and wants you to join them. Wha??? But, that's just not the way it works!!
I'm not saying your inlaws aren't awful people. I've completely cut myself off from some of my inlaws on my husband's side because I am NOT going to be a part of x, y, z. Period. Not okay. Not doing it. Which is my choice. But it's also done without animus. They're happy. I just never would be, living that way. So I don't. Except for large family gatherings that I can avoid them by being with others.