I am 5'10 and slept on the daybed and was fairly comfy. If your BIL isn't big, he might be able to use it instead of the boys. It's not quite the size of a twin. I could see the boys sleeping foot-to-foot on it, but not lying side by side...not wide enough imo.
Also, the room is fairly large, if you have an Aerobed, you could open one of those out in front of the sliders if you'd rather have the boys on something a bit larger.