From my own personal experience, remember this one phase as sad as it is "Nobody can screw you over worse than family"
I know your trying to help, but mixing family and business rarely works out well. Remember your going to become his landlord, not just be his brother. people's view of life gets distorted and weird (not in a good way) when family gets involved. Lets look at a quick scenario:
Brother has a bad day at work, normally would just suck it up and call it a day, after all he has bills and rent to pay. However, you are now his landlord, and his dear older brother wouldn't just put him out on the street, so instead of just brushing off the bad day, tells the boss to stick it and gets fired, and now can't pay rent. Dear older brother wouldn't put me out on the street, so he feels he has all the time in the world to find a new job.
Now your on the hook of either evicting your younger brother (if your parents or other family are still around, you can bet that won't go over well.) Or you now get to support your bother paying for everything out of your pocket.
Yes, I know, we would all like to think that family wouldn't do that to us, but they will and often do. I just went through this literally the past month renting out a room in my house to family. Needless to say, it is not pleasant, hurtful to everyone involved, and just an overall mess. Don't do it.