But if Bob moves Doug into his second house, then he's now either a) exploiting his brother instead of a stranger, which most landleeches are less likely to want to do, or (more likely) b) giving his brother a better rent to avoid exploiting his brother... and is thus making a much smaller profit than when he was exploiting the Ahmed family. And if not, then at least Bob only has so many brothers he can rent to, so the worst kinds of Bob will eventually have to sell off some number of properties.
But the goal of the immediate family exemption is to allow families to help each other own homes that aren't beholden to some shit stain developer or daywalking asshat. Specifically to let boomer and gen-x parents provide struggling millenial or gen-z children with homes, but I'm sure I could come up with a handful of other semi-likely relevant situations as well.
Back to your example though, the lack of profit he's now making pushes Bob to sell the place. And because all his landleech buddies are making less and less money per hoarded home, they're also more likely to be unloading their surplus stock at the same time. And if these people all lose money on the sale of their extra homes as prices come crashing down, fucking good.
And as prices come down, more people who are currently renting because buying is prohibitively expensive are able to stop renting and move into a place they own, this freeing up rental spaces.
Of course I'd also support going much more simple and literally forcing landleeches to give their hoarded homes on pain of prison time, but I figured that's a little more radical than the general population could ever come close to supporting.