When we age and especially after having children, our bladders drop. Maybe when Mom sits the bladder is being "squished" (can't think of a better word right now) and when she stands its relieving the pressure. You need to call her doctor about this. This could be the one of the reasons for her UTI and if not corrected she may suffer from many more. The problem means she is not completely voiding. Here's an article about bladder prolapse.
Just in case you don’t know the jargon, a bladder infection is usually called a Urinary Tract Infection, or UTI. In young women they can be very painful, in the elderly they can result in major behavioral problems. If you want to know more, go to Care Topics at the top right of your screen, click on it and then on U (for UTI) on the alphabetic list. It will give you articles, and many many old questions and discussions.
She went to a doctor and they found she had a bladder infection. So the doctor gave her medicine right away to treat it and she has to go to the pharmacy to get more of it.
https://www.drjohnmacey.com/blog/6-signs-of-prolapse#:~:text=Bloating%20and%20fullness%20in%20the,often%20confused%20with%20digestive%20issues.