Hello there and welcome to KA!
You've already received some extremely good info, so I won't add to it except to say that there was research done back in the early 90s by a Scandinavian female researcher. She found that it's possible that women don't fuse early because of our birth giving hormones. Our pelvises have to stay flexible to give birth, so our SIs don't fuse until we are coming to the end of our birth giving years. It's one theory. Hell, it's the only theory I've been able to find. When it comes to that, it's the only research I've been able to find that focusses on women and AS.
I'm glad you're kidnapping your mum to the AS clinic for evaluation. From the sounds of things, she's probably had AS instead of RA all these years.
Many hugs,
Kat
"I haven't failed. I've just found 10,000 ways that don't work."- Thomas Edison
