Yes, I understand what you're saying, but Su didn't say that the younger brother has a good job - just that he's working. No one knows if he has debts to pay off - such as student ones - or whatever, and as I said before, perhaps her eldest son's not really paying anything for himself either - just Su, being as though she's been invited into the home until they can afford a place of their own.
I see that she's just replied and they're going to stay where they are for a bit. I really think that's the best bet. At least if the worst came to the worst and they couldn't pay their way for any reason, I doubt they'd be thrown out on their ears as'd happen if they were renting their own place and something happened that they couldn't keep up with the rent. There's also Council Tax to consider, and all the other bills. Despite the younger brother not contributing, they've got the chance to save for their own place atm, and the sooner they get it, the better. It doesn't seen fair, but like I say, we don't know the full circumstances.