OP here. Here are the three scenarios that I was thinking of which caused me to create this thread. They're all somewhat different situations so I don't know if any universal behavior should apply to all situations.
We live in the U.S. and my adult cousin lives in Canada. She was engaged and planning her wedding. Our uncle (he's her mother's brother as well as my mother's brother) had a young daughter and she was going to be the flowergirl. It was a big white wedding, many months to plan it, etc. Uncle, his wife (AIL = Aunt In Law), ChildCousin (flowergirl) and two other children purportedly had passports and plane tickets. A week before the wedding, all was supposed to happen as planned. When we arrived in Canada for the wedding, only Uncle was there. AdultCousin (bride) was shocked and angry but is classy so she didn't give me any details. All I could tell was that somehow things had changed last minute and AIL and ChildCousin weren't in attendance. Several family members asked Uncle what happened and he was tight-lipped.
At this time, I lived 150 miles from Uncle and AIL and I would make the drive to see them once in a while and spend time with them. So I wouldn't say we had a super close relationship but it was a good relationship.
I got engaged the next year. Upon my engagement, AIL contacted me and asked me if ChildCousin could be my flowergirl. I didn't know more details about the situation/history with AdultCousin's wedding so I said yes and we discussed wedding plans throughout my 10 month engagement. This included that ChildCousin would be in wedding party photos, at the rehearsal dinner, have a special dress, etc. Uncle, AIL, ChildCousin and their other kids all RSVPed yes to the wedding. Less than one week before the wedding, AIL emailed me and said that she and the kids wouldn't be attending because the wedding was during the kids' nap time! I was polite but firm and wrote her back and said that I was angry about that. Only Uncle attended the wedding. He didn't say a word to me about AIL or ChildCousin's absence.
A few weeks after my honeymoon, Uncle asked me if DH and I would like to go to their house to have dinner. I didn't know what AIL had or had not told him about her absence from my wedding so I wrote out what I've told you here. That she had been the one to ask me if ChildCousin could be a flowergirl and that she'd backed out at the last minute with a lame excuse. He didn't reply. I got silence. And I didn't hear from him or speak to him for a long time after that. When we've seen each other at family functions (AIL's generally absent) we have been friendly but haven't brought up the topic.
I've been married nearly 10 years now and just a few days ago, for the first time since, Uncle contacted me wondering if DH and I wanted to go to their house to visit. I know he has other motives for why he contacted me recently which are unrelated to all of this so I won't go into that. But it got me thinking... I never blamed AdultCousin (bride in Canada) for not telling me what AIL did to her. It wasn't my business at the time. But I think I had mentioned to AdultCousin during my engagement that ChildCousin was to be a bridesmaid. I wished she had warned me then not to trust AIL. When another cousin got engaged the year I got married, I did warn him and his bride about AIL's tactics. I'm wondering now if Uncle should have told me during my engagement what had happened during AdultCousin's wedding. If he suspected that his wife would back out after I'd really done a lot to accommodate their small children to be guests at our wedding, shouldn't he have warned me? Or at least apologized after the fact? Or at least replied to my email?
That got me thinking about other offenses that others have made and how spouses handled it. Which brings me to...
New cast of characters. DH and I were good friends with Betty and Bob. Several years, Betty's mom, Ava, invited us all over for Thanksgiving dinner. There were usually about 15-20 people at these gatherings. One year, Bob and DH had a great time well after dinner staying up and watching lousy movies on TV. The next year, we were at Ava's for Thanksgiving again. At some point after dinner while we were all cleaning up and moving from the dinner table to other rooms in the house, DH came to me and said "Bob and I are going to go to the video store and get some movies to watch late tonight." I was concerned because they were leaving mid-party to go do something that if they'd had an iota of forethought they would have done before the gathering. DH said it was Bob's idea, indicating that as Ava's son-in-law he knew what would be ok. DH assured me they were just going up the street and would be back shortly. To my recollection, they were gone about 30 minutes. It was kind of chaotic because there were about 20 guests this year and we were cleaning dishes, putting leftover food into containers to take home to various homes, etc. By the time we all sat down again to just chit-chat, DH and Bob had returned.
A few weeks later, Betty told me that Ava was very upset that Bob and DH were gone so long from the party. She said they'd been gone an hour. I agreed with Betty that it was very rude for any guests to leave in the middle of a party to go do something else "fun" (as in not an emergency) that should have been planned for another time and then return later. But I hadn't thought they were gone quite that long. She insisted that Ava was very angry. Ava had never said anything to me about it but I told Betty that I didn't approve of what Bob and DH did and that I was sorry on DH's behalf. I talked to DH about it later and he said it wasn't a big deal. DH wasn't raised in a home with much social consideration or refinement so I told him that I strongly believed that he and Bob had been out of line and that they shouldn't do something like that again. We went round and round about it for a while. DH never said anything to Betty or Ava about it, nor they to him.
Same Betty and Bob from the last scenario, only the tables were turned this time. Our house, DH and I are hosting a milestone birthday party with a small number of guests. There were other transgressions that day such as Betty bringing uninvited guests to this gathering, who didn't really know the guest of honor, when she knew that I wanted it to be small and I had purposely restricted the guest list.
But the issue that really angered me was the following. Bob and another guest, Cameron, had wandered around our house, in rooms where we were not expecting guests to be, certainly not holding any party events. I hear that some children are banging on my piano. I always wash my hands before I play my piano because I don't want sticky, sugary keys. Getting food between piano keys is a great way to ruin an instrument. I go in there to see that Bob and Cameron are standing there watching their children with cake-frosting covered hands thinking nothing of it. I was incensed at the potential mess on my piano. I was incensed that people thought it was ok to wander around my house letting themselves into other rooms and opening piano fallboards to entertain themselves. I'm usually lousy with confrontation and let things go even if I'm angry (see the previous paragraph - I did nothing about the uninvited guests). But I raised my voice and told the kids to go wash their hands.
Betty, Bob and Cameron had all known me long enough to affirm that I'm usually easy-going. I usually don't get irate or voice objections. They'd known me for years and I'm sure they'd never seen me that angry about anything. Just for that alone, I think they should have at least said "Danika, I may not understand why you are angry, but clearly I've stepped out of line. I apologize." But they did not. They treated me like I was out of line for defending myself. Later, when I talked to Betty about it, instead of apologizing, she tried to gaslight me and accuse me of being "too particular and too attached" to my piano. When I also told her that I was upset that she'd brought uninvited guests to the party, she said nothing. And the next year tried to bring more uninvited guests to another party I was hosting. But I shut it down before it could happen that time (that post is somewhere here on EHell).
I hope those were entertaining reading, at least.