My mother has been living with my husband and I for 2 years. Her personal hygiene is just awful. Her not changing her wet underwear is the worst. I know I sound like a broken record to her, but I do care about good hygiene. Showering is also a problem, I have to ask all the time. I got rid of her underwear and bought underwear that has a bladder pad and panties that are nice looking as well. She does not want to change them after they get wet, then will let them dry and put them back on, if I don't catch it. I am getting pretty good about that though.
So, when she took a nasty fall trying to walk into the hospital to visit Dad, and smacked her forehead and nose on the concrete, the next day, I insisted she shower, proclaiming the comforting benefits of warm water on her sore joints after that fall. It worked - that time, at least. I did struggle with her for the rest of her life to get her to shower. She insisted getting the tub made her feel she would fall and it was too hard to step over the side of the tub. So I spent $800 cutting the side of the tub down to make an opening for her to simply step through. That worked for about 2 weeks- then right back to the resistance again. Even in the nursing home, the staff had to bribe her with sweet treats (not my choice of bribe!) and such to get her to shower. They finally got her to use the walk-in tub, and she *loved* that!
Bottom line: You can't physically force her to bathe - but you can bribe - and maybe that will work. Find something she really likes and tell her she can have it after she bathes. Be gentle about it - sometimes it's a pride thing -they don't want you to help them or see them naked - but it has to be done.
I know this may not be something you want to do, but can you "guilt" her into changing the underwear or letting you help her to do it? Tell her that if she develops an infection due to the wet underwear not being changed, the doctors are going to want to know why it's happening and may report you to APS for not taking care of her properly, and if they feel you can't care for her, they may recommend placement in a nursing home. (Please don't take that the wrong way - I am *not* saying you don't care for her properly.) I had to take this tactic with my mother when all other efforts failed. I had to tell her, "Mom, I don't like having to say this to you, but there is an odor in the house because your underwear is staying wet with urine. Please let me help you keep clean, so that you don't get an infection from it. It's not healthy for your skin to stay wet like this. If you develop an infection, I will have to take you to the hospital, and they might say I'm not taking care of you well enough and want to put you in a nursing home."
I hated having to take that step, but it worked for a while to get her in the shower. There were also times I had to be downright firm with her (to me, I felt I was being mean to her and I hated it). She would put on this little girl voice and say, "Must I shower??" and give all kinds of excuses - she would shower after this tv show as over. Then it was after lunch. Then dinner. Then it was too late, let's wait til the morning. At these times, I would look at her and say, very firmly, like you would to a child resisting a bath: "Mom, there is an odor in the house from you not showering, and you are not the only one living here. It is unfair that I have to live with this odor when I am here to help you bathe. You are taking a shower today, and that's all there is to it."
I absolutely hated being like that with her, because to me, I felt like I was being disrespectful and mean to her, but I wasn't yelling or screaming at her and I didn't physically force her to shower - but this was the one thing she would respond to when all other attempts failed.