It's one of those unknowns.
My mum had rheumatoid arthritis from her late 50s, and osteoporosis. Hers wasn't treated, and compared to your mum was clearly very minor, given that she lived to the age of 93. There was plenty of evidence of spine curvature, lost height and ulna drift as she got older. She was well 'ard, though, and mostly suffered in silence.
I've got osteoporosis and RA as well, but given that (so far as I know) none of my 4 older siblings have, I can't think of a reason for me having osteoporosis other than a hereditary factor, but of course that doesn't seem to have affected anyone else.
Compared to my mum, I'm a dreadful wimp and go running off to the doctor, whereas she couldn't be bothered.