I first want to acknowledge the hard work that you are doing. Taking care of children along with your mother-in-law must be very hard on you. Would it suprise you
to learn that the development of incontinence is one of the major reasons people are placed in the nursing home? This is because it is not easy an easy problem to deal with!
First, I would ask you to take your mother-in-law to her health care provider , tell them about her incontinence, and see what they can do to help her. There needs to be an investigation into why this is happening to her. Many times, incontinence can be made worse by medications, diet, and constipation, so she needs an entire health review. She could also see a specialist in Urogynecology. They would do a more extensive exam of her pelvic area to see if anything there is causing her problem. They can also recommend medications and treatments to help her incontinence, which may lessen or eliminate the problem all together.
As for you taking care of her at home, it sounds like she will not allow you to help her yet. If she would understand, tell her you know that incontinence is embarrassing, but she needs to know that this is not just her problem, especially since she lives with you and you do all her laundry. Tell her that you are going to help her look for a solution (by taking her to the doctor), but she needs to help you by wearing the pads or briefs. Hopefully, you can work together to make this problem bearable until a solution is found.
Maybe your mother-in-law and you have a strained relationship, which may be another factor in this problem. Perhaps hiring a part-time caregiver (temporarily) will help. Sometimes, an outsider who has experience with dementia and incontinence can find solutions that you never would have considered. Also, to be honest, your mother-in-law may listen to a caregiver tell her what to do, as she is not "family". I know caregivers are pricey, but you are already stressed and this may offer you a few hours of peace and sanity.