My mother-in-law is able to take showers and clean, but is failing to do so. What should I do?

Fedup, call a professional caregiving Agency, see if they have a "bath aide" who can come over and help your mother-in-law. Your mother-in-law might be more receptive to a bath aide then having you or her son help her with a shower. It would be worth the cost.

Please note, as one ages using the bathtub or shower can become scary. There is that constant fear of falling. And some elders become claustrophobic if the shower stall is small. And some elders don't like how the water feels coming out of the shower.... and if using a bathtub, having issues trying to stand up to get out of the tub.

Does your bathroom has grab bars on the walls? How about a shower chair? Is it usually friendly for anyone who has a knee issue?

Does your mother-in-law wear Depend type garments?  If yes, have her change them frequently.  Open up some windows in the house to let in fresh air.   Wash Mother-in-laws clothes using some 20-Mule Team Borax. 

And remember, some day we will become either elderly or have a knee issue... and someone will complain about the odor :(

Have your husband deal with her for she's his mother.