oh for goodness' sake, this showering daily stuff is way overrated. it's only a product of modern plumbing opportunities, it has little to do with whether most people need an all-over shower. most elders grew up in a one-bath a week culture (which of course may have led directly to the invention of deodorants).
the skin of most elders is too fragile and dry to need daily wash-down by outpouring water. three times a week is great. otherwise, they can do what most cultures have always done, including our own. wash-cloth, soap and water gets the job done without so much stress.
plus, id say EVERYONE with dementia is disturbed by showering. latest research shows that brain wave function seems affected by the beating of water rhythms on the skull.
1. stop bullying Mom (that's one BIG thing that's upsetting her);
2. get a hand-held shower, beginning from the feet up, talking soothingly to her. "There, Mom, this'll help those aches and pains you were telling me about..." blah blah blah. Keep this to not more than 3 times a week;
3. if you guys can't be nice about it, then hire a bathing aide and i'll bet your Mom does much better that way. (we KNOW how to get your Mom feeling comfortable with us);
4. washcloth and warm water in the bathroom sink and some gentle cueing for Mom the rest of the time.
Big time: be NICE, be KIND, be persuasive, suggesting and bribing NOT bullying. And let go of the moral judgment stuff.