Horrible situation for your SiL and your DiL, ExD and very sad - especially for the MIL.
In no particular order, you can do many things but are not going to be permanently rid of "the stink" until you do something positive to change MIL's circumstances.
I think your family should contact MIL's GP to share the problem and to find out what possible/additional care could be provided for her. You don't say whether she seems bright and aware still, or whether she might even be displaying symptoms of dementia. Two things stand out here - she sits in a chair, without pads or underwear and pees; and she refuses to sleep in her bed. These two things might be just old lady stuff or determination to exert a bit of control, or she could be thinking that peeing in the chair is a bit like visiting the loo and she's just anxious that if she lies down sleeps - she'll wet her bed. Is she fairly ambulant still, or is the chair the place where she spends most of her time? Does she respond to and retain normal conversations, or does she seem fairly oblivious to life around her ? Is the incontinence the only issue or are there other worries for her family?
Please on't waste money on buying a recliner for her to use. If she is going to continue living at home, buy an ordinary plastic covered armchair which can be sanitised daily. A recliner will only encourage her into thinking that she can sleep there and that is harmful to her health. Sleeping upright at nights in a chair should be totally discouraged and as she does not seem too aware of the rights and wrongs of life at present, she is likely to spend most of her time there, develop all sorts of skin problems and circulatory problems which will then put her health at risk.
Chairs, beds, everything, as has been discussed earlier, can be properly steam-cleaned and sterilised and rendered smell-free by numbers of companies that are used to working predominantly for nursing and care homes. If items are properly treated by them, you should not need to be using deodorisers - since there should be no ongoing smell. As GrannySomerset states, you will definitely need to find the most comfortable and correctly-sized incontinence pads and she must keep them on and have them changed regularly.
Adult Social Care perhaps can be approached so that she can be assessed by them or a CPN, not only for future needs but to establish whether she is safe still living in her own home. Carers, however good, and especially when given a high NHS workload, sometimes almost operate on a "drive-by" basis....they have little time (sometimes 15 minutes!) to make drinks and get clients up and fed etc., without cleaning and trying to deal with someone who clearly can be a very difficult old person. An assessment will show whether she now needs more care than she is having. It's highly unlikely that she would agree to going into care - so, sadly, someone else needs to be brought in for support and involved in a professional capacity, before your DiL loses the plot completely!
Good luck with it all. Be determined, start with the GP.