My sister-in-law is caregiver for her physically challenged husband who has diabetes, heart damage, obesity and is barely ambulatory. He walks with a walker, but falls quite often and she has to call her son home from work to get him off the floor. When he feels the need to go, he hurries toward the bathroom and usually trails urine all the way to the bathroom and often soils his clothes before he can get on the stool. His wife is worn to a frazzle from cleaning him and his clothes, getting him redressed, and then going back and mopping the floors and cleaning anything else that was soiled. He is 80 and she is 79 so she really doesn't need all the stress. Other than outside help, which they both resist, how have others handled this situation, or at least made it manageable?
It sounds like BIL may fall because he's rushing to make it to the bathroom. He may be waiting too long. Have they considered getting a toilet chair for him? My husband had one. It was bariatric, which means it was heavy and wide with railings for safety. That way he wouldn’t have to go very far to use the facilities. For privacy, they could purchase a simple folding screen.
I know exactly what your SIL is going through. It’s not pleasant. Could your husband speak with your BIL and suggest he use the toilet chair?
I found that Sam's club brand didn't have any plastics around the legs to cause any irritation.
I would try using the tab kind inside of regular briefs and don'tuse the tabs. Then I would tell him to stop taking off and making a bigger mess. I would also set up a toileting schedule. Set an alarm and every hour or two he gets reminded to go use the toilet.
Get washable chux and cover his chair and bed to help keep it contained.
Good luck, such a difficult situation.