Finding the right place for 4 people will be a challenge.
In parts of Europe it is common for a multiplex to house many generations. I know a family where the parents are on the main floor. The daughter (family of 5) is in the middle and the son is upstairs. I believe the parents bought the land and built the shell and then the children were financially responsible for finishing the inside. The son finished his space about 6y after the daughter. (I think he lived on the main with the parents until he was ready.) His new place is 3 bdm too, so there is room to grow. On title I am not sure who owns what. It would not be uncommon for the parents to own it all but the children will inherit the floors when the parents pass. That is 8 people comfortably living on a lot no bigger than most single family homes. The grandfather needs constant care so it is good to have family close by.
I am a big fan of multi-generational family living and I think there should be more options and better zoning in North America for this. I have dreamed of a place with a central shared kitchen and a wing(s) for my parents and inlaws. So I get your desire to help them.
However, taking on the financial burden of living expenses for 4 people is a huge undertaking. I am not sure of the best way to do it without loosing your own shirt.
I love the European tradition.
I've just taken care of 2 relatives, both who could no longer live independently. It was VERY hard. I completely applaud your efforts to take care of people that love you, but I urge caution. It's hard. So so hard. You said they are picky. UGH... that's a bad start!
It should be a labor of love (sounds like it is) and not an investment. We actually upsized and bought a huge house to move an elder in. She had the master bedroom suite... bigger than her little senior apartment, and all her familiar furniture in it. She contributed a portion of the house payment...which was actually far less than her assisted living apartment, so it actually was a good choice financially to make her money last. Those care places cost a fortune, AND she wasn't getting the care she needed. Plus she was lonely. I was going by the senior place almost daily, and feeling spread thin. She was absolutely 100% on board with the move, and that was a crucial component. She lived here until she passed away. It was very restrictive on us at the end, due to the round the clock care involved, but I would do it again, with out a doubt. It was a gift of love to my dear gram.
Then, a few years later, we moved my mom in with us after some alarming things happened to her during independent living. She had dementia, but it took us a while to figure that out. She'd been in her house for almost 40 years. With mom, it was a bumpier road to be the caregiver and have all of us under one roof, while dementia set in. We loved it, but it called for more diplomacy. After a year or so, we bought a little house down the street and got caregivers to come in. That mostly worked out until the dementia got really bad. She needed 24 hour care and supervision, and we moved her to a dementia senior care facility. She thrived there, was safe, well fed and social.
What amazed me in both care situations was how fast they changed. Just when you think you've figured out a great plan, their skill level and needs change. It was tough.
Unless taking care of all 3 of them is part of your long term plan, I'd use caution. You may well be the one to take care of them when it's no longer a choice. But, for now... Do they think this is a good idea? Do they get along? Seniors can be blunt under the best of times, if illness, physical handicaps or dementia set in, it could be a rough go for you, especially if you don't have a backup plan or help. Do you do all the shopping? Who cooks? Can they drive or need to be driven? Where are their doctors? What about the all important social life-- can they still get to activities and friends from the new location? Keep their world as large as possible for as long as possible. I'd really talk it out with them, your brother, and family. Think through the "what if's". Perhaps help, but not do it for absolutely free, because it might be better for everyone to feel like and be a contributing household member. It sets a tone/boundary for the arrangements so you and your brother get a little respect and so do they. Maybe what ever they can actually afford, minus 20%. Let them buy what they want at the grocery store with their own money, whether lobster or mac and cheese.
Also, the senior center in your area might be a great resource. Services like Meals on Wheels can be great regardless of where they live.