As disgusting as it is to cheer someone on to kill themselves... I can't imagine anyone going to jail over this, or being successfully sued. If he didn't like the posts.... he could have just stop going to Reddit. That was always an option. It's not like the Reddit trolls made him kill himself - he came to that decision himself and merely informed them. Yeah, the insults and the egging on didn't help, but they also didn't start his decline into suicide.
His ex-wife probably wasn't responsible for his death either. Sure, she insulted him, but insults alone don't cause people to commit suicide. Depression and isolation cause suicide. If you're going to sue people for causing his death, then conceivably you could sue just about anyone in society as society didn't do "enough" to save him. You could sue the government for not stepping in and saving him. You could sue the neighbourhood for not checking up on him. You could sue his parents for not supporting him. And what of his sister who is bringing this lawsuit? Why didn't she save him? Maybe she should sue herself!
It's understandable that a family member would look for a reason for why someone committed suicide. Surely, they think, someone or something else must be to blame. But sometimes people commit suicide due to depression, and that's no one's fault. Sometimes there is no one to blame for a suicide. The Sister is lashing out, looking for someone to blame for his death, because she can't come to terms with the fact that sometimes people kill themselves for deeply personal reasons or feelings. Accepting that fact would almost be akin to blaming her brother for his own death, and she doesn't want to do that, so she seeks to blame someone else.
Suicide is always tragic (except in Hitler's case), and while sometimes others ARE to blame, sometimes there's no one to blame. You can't blame the suicide on some nasty forum posts. If that could drive anyone to suicide, they were already on the brink anyway, and it's not like we can go around wearing placards saying "I am emotionally fragile, treat me extra special today!" on the internet.
I don't go to Reddit or 4chan precisely because the people who are on those forums are often (but not always) nasty, sub-human scum and I don't want anything to do with them. He had the option of closing his browser window. It's not like highschool facebook trolling where the trolls actively harass you offline and online and follow you around. He had the option of simply.... not going to those sites.