Oh no Evian, that's awful!! :-( I do hope he's ok.
In my (very limited) experience, it depends on what he overdosed on. I know someone who overdosed on painkillers, and it just made them very very sick. They did have to go to hospital, but they didn't 'do' anything like stomach pumping. I suppose it depends on the severity of it though.
With regards to after care. I think it will all depend on how your sister feels. The person I'm talking about above just had a "do you think you'll do it again" kind of conversation at the hospital, but it wasn't with any kind of mental health worker. Just a nurse.
I would hope that, given he's obviously very troubled, they'd offer a bit more help than that, but I don't have much faith I'm afraid.
If your sister or another family member were to talk to them about the possibilities of making keeping him in, maybe under the mental health act, then it's their job to listen to these concerns.