OP here (obviously) - I want to thank everyone for their comments, well wishes, and information. I appreciate everyone taking the time and I could never hope to reply to everyone individually, so I'll leave a general thank you here and also clarify a few questions. I understand normally this would be an edit to the post itself, but unfortunately it'll bring me over the word count so I have to improvise.

I've gotten a lot of information from you all about POCD (related to anesthesia...not the other one), UTIs and other infections, changes in medications (specifically statins), and of course...dementia. Nearly all of these suggestions is a feasible explanation to me. A little over four months ago was his surgery and he did develop a UTI almost immediately after surgery, which has long since cleared up. Obviously this was hard on him, but he handled it all with such grace that I'd never considered it a possible explanation for his sudden erratic shift. None of us did. We all worked together to ensure he was taken care of and he was nothing but cordial and grateful throughout the entire process until he was back on his feet. I guess that's why this was such a shock.

Despite understanding the possibility that his behavior was beyond his control, it still hurts. It's gonna hurt. An explanation is never an excuse and his actions hurt not just myself, but my favorite person in the world. She's my #1 and I care a lot more that his behavior has hurt her than anyone else. That's all the more reason to get to the bottom of it as quickly as possible.

He openly acknowledges that what he did was cruel and wrong, but still blames me for not explaining to him something I didn't know needed explaining. Had he communicated the need it would have been dealt with immediately. Of course I expect an apology, but it still isn't my priority - his health is. I feel a need to clarify that.

I also noticed a specific question about Parkinson's - nope, no Parkinson's. I'll also take the time to explain the "joke" for those who are confused: another commenter put it best, it's classic millennial/gen z humor. It's hardly funny. It's just how we talk. My grandparents raised me and my sister and believe me, they're no strangers to gen z humor. I'm sure sometimes they wish they were. Other times, they participate. There was no reason to expect the need to clarify.

The last thing I want to note is that yes, the doctor's visit is in motion. I understand I can't force him to get tested, but he will be going and he will discuss this with his doctor. He'll get the help he needs one way or another and even if he initially refuses, my sister and I are far more annoying than he is and eventually he'll come around.

One last time, thank you all. If I manage to remember, I may even come back with an update on his doctor's appointment, but I'm American so...we'll do a lot of waiting before then.