Mom doesn’t understand it's okay to soil her diaper.

I agree it would be better for her to get up but she says the pain in her hip is unbearable when she tries to get up. She has a bad hip which probably should have been replaced years ago along with a bad knee but for her own reasons she refused any treatment for both. Anyway, my daughter was actually able to get her up and moving for a little bit on Monday. I told her she worked a miracle bc I have been trying for the month she has been “bedridden” to get her up and moving. She can be a handful when she makes up her mind about her what she can and can’t do. I think I’ll ask my daughter to come over every few days and see if she can get her up.
Ah well, such is life. 🤷‍♀️

jjcares, reading your profile was exhausting, so I can't even imagine how you even get out of bed in the mornings. You asked the question here, "How do I get her to understand it’s okay?" regarding her deteriorating bathroom habits. But the bigger question is: how much more can you endure? Or, what do you consider a solution to your current situation, which will only continue to get more challenging? Your first obligation is to your husband, not your parents. Your parents have had their whole lives to prepare their wallet and minds for this eventuality. Your parents can both transition to a care community where they can both get what they need. This will help you as well as you. You did say that you live in a rural area where your husband's medical care is about 100 miles away. This is also not sustainable in the long run. What would happen if the caregiving got so overwhelming that it impacted YOUR health and then who would help the other 3? Please consider the upside of downsizing and moving closer to the better care that the 3 need. Yes, this would be a major change. But can you imagine a daily life where you and your husband can have much less stress and your dad is getting healthy social exposure and your mom is getting the care she needs? You will actually have a life. Your parents sound like they will need to apply for Medicaid. Your name is on the house deed, so maybe a 1 hour consult with an elder law attorney so that you and your parents can be advised of the best and quickest pathway to this end would be money well spent. Please understand I'm not making this suggestion flippantly. I totally understand it would be an enormous change. But if you don't make a move now you/your family WILL eventually have a crisis that will force a change and then you will have less control, under duress, in a contracted time frame and fewer options. Think with your head and not your heart so that you can move towards a solution that works. I wish you wisdom in your mind and peace in your heart as you consider your options.

Can you use a bedpan for her?  Her mind's working enough to know she's got to go.  No one wants to lie in a wet, soiled diaper, and that would cause other problems as well if her diaper was not changed right away anyway.
I saw this: https://www.handicappedequipment.org/bed-pans-for-the-bedridden/

You might check Senior Care Products at the bottom of this website and your local stores for similar products.

If you can't get her on a bedpan, there are bed pads used for incontinence and maybe that would be an option to slide under her wherever she is, then remove.

I also saw this for "bedridden and continent" which may be an idea:
https://www.independentliving.co.uk/cp-editorials/toileting-in-bed-incontinence-protection/
Looks pretty involved, but maybe there are other similar things.

Others here will likely have more ideas, too.

You probably won't bring able to get her to understand. It is a life time habit, after all. It would be good for her to get up and move.