Harvey knows about the Tatsyrups
While Val wants to keep Benjamin at the Dentons' house because she wants a son in her life, Harvey also tries to keep him at home (through his many rules and customs) just to prevent him from going on the hike--because if Benjamin goes into the hills, he's bound to drop by the local shop. Harvey may be the only local who knows what Tubbs & Edward Tatsyrup do, and he (being obsessed with order in his life) can't be bothered with the trouble & stress of reporting them to anyone else. It seems to be only partway into Series 2 that Harvey gets the "keep Benjamin as a pet" idea, so until then Harvey is right in claiming that he cares about his nephew, and he's inconveniencing him for a valid reason.
This also ties into the fact that, in Series 2, Harvey alone could explain that the Tatsyrups weren't causing the epidemic, but, in the irony so typical of Royston Vasey, he's locked in his basement at the crucial moment.