A couple years ago, Reddit user gfjq23 got into a Facebook argument with her sister over whether or not Santa Claus exists. She became concerned over her sister's reaction, which included blowing the issue way out of proportion. It was so bad, OP's sister got medical tests done - and it revealed something unexpected.
Original Poster had asked Reddit for advice at the time and then provided an update.
Submissions have been edited for clarity, context, and profanity.
To summarize the last post, I posted a Ryan Reynolds meme about Santa Claus not being real on Facebook which my 13-year-old niece saw. My sister flipped out about it and wanted me to publicly rescind and say how Santa is real, but I thought my nieces were too old to believe in that stuff and refused. It lead to a crazy fight between us. Link to the original.
Anyway, I talked to my nieces and neither of them believe in Santa, so they were baffled about the fight. I talked to my BIL and he said my sister has been flying off the handle lately. We agreed she should probably get a check up and he convinced her to go to the doctor.
Onto the update. They did a MRI and nothing showed up. Then they did some bloodwork which looked fine, except some elevated cholesterol. She isn't pregnant. They pretty much wrote her off as a crazy person and sent her to a psychologist for stress. After a session, the psychologist told her to do some "deep breathing" and sent her away as fixed.
She got worse. She stopped sleeping and barely ate, yet still gained weight. Any small annoyance would send her into a rage. Commercials were making her so upset she would ugly cry. I asked my BIL if they tested hormone levels or anything like that and he said the doctors didn't feel it was necessary.
She called me one day crying and apologizing, saying she was the worst sister ever and I had every right to hate her. She was so devastated she ruined our relationship and such. It was weird and NOT my sister, so when I got a chance to speak I told her she was going to go see my doctor and I wasn't taking no for an answer. I set up an appointment and my doctor ordered a full blood panel including hormone and vitamins before my sister drove to town for her appointment.
When my sister drove up we spent the morning shopping and she was unpredictable. One minute she was happy and the next yelling about some perceived sight ("That fucking pretentious makeup counter bitch just looked at me funny for my cheap drug store makeup."). It was uncomfortable, So I just walked on eggshells to keep her from exploding.
Anyway, results of the bloodwork and a good doctor: perimenopause. Her hormones are completely abnormal. None of her doctors would even consider it because she was "too young" for menopause, so they didn't even bother running the tests. She'll be coming up with a care plan with my doctor for hormone replacement therapy and diet change to hopefully get back on track.
She still a nutcase right now. For example, she called me crying the other night because she will never have more kids (wha...her husband had a vasectomy years ago). I'm driving to her place next weekend and we're going to batch cook a bunch of meals for her new diet plan (I'll be doing it with her as I could stand to eat healthier). So it'll be a slow process, but we have a diagnosis and plan. I'm just taking her outbursts as "crazy hormones" right now because it'll take awhile to even out.
I got her a dark chocolate cake for Easter that said, "Happy Reverse Easter (when the Easter Bunny takes back your eggs)" because I'm kind of a jerk. She thought it was hilarious though, so we are good.
TL/DR; Sister is going through perimenopause, so she's irrationally, but understandably nutty right now. Oh, and Santa Claus still doesn't exist.