I have my reasons and need to know?

If 2 people are using the same Wi-Fi ( at home ) can you spy on their i messages, whatapp and the like from and iPhone 3GS without them knowing, yes I'm suspicious and want to make sure.

No you can't.

If you have basic hacker skills, yes you can. Having direct access to the network makes this pretty easy to do really. Wi-Fi can be secure, but not to someone on the same Wi-Fi legitimately. It is possible for hackers or government agents to do this kind of thing remotely by hacking into your network and utilizing various exploits that depend on computer companies leaving holes in security software. If you use windows, you know how many updates they constantly want you to download and install, this is the reason, they are patching holes they didn't know was in their software because a hacker found a hole and reported it. Almost all software and hardware that can connect to a network must be patched in this manner. Your, oh so safe and private data is a fantasy. So yes, this can be done, not by me, because I'm not a sociopath, but there are people that can do this quite simply.

Shiggenblugg is right, people can do it but generally would need access to the other phone to set it up in the first place as you would have to link them. But all someone would need for apps like whatsapp is your username and password