Have you all sat down together and looked into every possible option? Have you involved any other agencies, such as social services, Age UK, Alzheimer’s telephone helpline? It can be useful to get an objective opinion and some forewarning of the impact that this could have on your brother, sister-in-law and their children.
This may sound brutal but your dad is going to get worse, not better. It’s a massive undertaking. My dad got to the point where he couldn’t be left safely alone for 5 minutes. The impact it had on my children (and work) was huge, and he didn’t even live with us, he still lived at home with my mum. It took a crisis for him to go to respite, where he has remained ever since, safe and well/cared for.
I’m not saying that our situations are the same, just that you all as a family have a duty to each other to really know what you’re all agreeing to. There are many options open to you, and lots of support out there, you just need to search for it, and keep asking until you get the answers you need!
Good luck to you on your journey, it is long and hard but I can sense in your post that you are trying to get the best in place for your dad and your family. Just don’t let dementia rob both your dad and other family members of their health because caring is a long, exhausting road. Good luck x