Sounds like a suitable set-up. Just a word of caution though.
A friend of my DD's had a similar arrangement at this age (though unfortunately because she had too many problems living at home, mum put new DP's needs above hers). She moved in with her sister who lived nearby and was at Uni. The older sister was very responsible, however SS got involved as they were concerned about the other people living in the flat and all the coming/going of students. So she was found a foster carer (who has been fantastic and actually kept her until she finished her A levels).
So maybe ensure school knows of the arrangements and that there is nothing that SS could be concerned about. I think you're very lucky to have such a lovely older son.
We've talked it through at length and I wouldn't have considered it if the older boy didn't want to take on the responsibility. They have always been very close. Unlikely that the youngest will stay on at school, I am expecting him to want to be with me when he finishes next June. This move is likely to be 3 yrs then we will be going abroad.
I've got to move 200 miles north and my 15yr old (yr10 high school) has flatly refused to move (girlfriend) so he is going to live with his older brother and finish his gcse's at the same school. I think I've covered all the bases but I am still nervous (guilty) about the set up. My mum lives 15 mins away and there is a step sister across the road. Anyone else ever been in a similar situation?