I hear you OP.
I haven't read the full thread but I am sitting here right now thinking the exact same thing.
There are lots of reasons which I don't want to go into, but an example (which happened a few years ago now) is, a sibling has a DC. They've not been a great parent and have been absent a lot. Our mum was hosting DC's birthday, a pretty important one.
Instead of coming round as planned, they showed up extremely late and hungover, because they had recently had a row with their toxic partner and "made up" on DC's birthday, so they had whisked away to a fun road trip and got extremely drunk/high. Showed up hungover and sat drinking.
The reason this example comes to mind is I remember seeing the doubling down. Me and my mum were not outwardly saying anything but were obviously a bit "WTF". Instead of being embarrassed or contrite, sibling kept attempting to show photos of their lovely trip and laughing quite derisively whenever my mum naturally wasn't super interested. They kept going with mentioning the trip and showing photos until my mum felt too awkward to ignore.
They were only interested in talking about themselves. On their DC's big birthday. And when people were off about it, they just doubled down and pushed harder.
I don't understand it. There are many far worse examples of this sibling's behaviour that I don't feel comfortable sharing.
It's really painful to me and I'm constantly baffled and upset by the antics of both people relatively close to me, and strangers.
I think I need to specifically seek out stories of people being kind and doing good things, because I see so much bad these days. Everybody is struggling more too and I think that is making things worse.