Is this a sign of dementia? My 92 year old father-in-law is making mayonaise, peanut butter and dill pickle sandwiches.

I think the more important thing is that they are both getting the nutrition they need. If his wife won't eat the sandwich, then she needs to be able to have an alternative. If he can't provide one, you should probably do it.

I wouldn't mind so much but he splits the sandwich with his blind wife. She has asked him not to add the extra ingredients, but to no avail. Should we take over sandwich making duties or let them be?

My mom used to make those for my dad's lunch like 30 years ago when they were much younger. I thought we were the only ones! I still find the combination gross, but they were perfectly "normal" back then. It takes more than a strange sandwich to meet the criteria for dementia. If he likes that kind of food, I'd say let it be.