Dementia brother has problem cleaning himself after bowel movement. He also creates quite a mess on the floor when urinating. After some research it has been suggested that he sit rather than stand when urinating. Further research yielded info on a bidet toilet seat that might help with getting him to sit as well as help with cleaning himself. Although his dementia is not too advanced at this point, he does require an assisted living facility and I'm not sure he could cognitively operate a bidet system. Anyone had experience with an easy to use bidet seat installed in place of regular toilet seat.
As for urinating, missing the toilet is very common. There are tricks to help him identify where to aim, such as lights that go inside or different color schemes. You can search this forum for more ideas. Others get a urinal to keep by the toilet and pee in there and then dump it. Sitting down may work or may not, depending on what shape toilet you have and other factors. Good luck!