Can your PC get infected with malware by an infected PC sharing the same wi-fi network as you?

Theoretically, yes. This is what a computer worm does. If the computers can see each other on the network (they can ping each other or they show up in the Workgroup view) and your computer has an unpatched vulnerability, then it could theoretically be exploited by a worm and infect your computer.

No, that will not happen. 

o. It does not work the way you described it.  The virus itself would only be able to infect and replicate IF you EXECUTE the file itselt. An infected computer sharing files does not infect others unless the user decides to execute it.  Ultimately, it is the humans who are a danger to computers.  A computer virus is basically a software that was programmed to do what it was intended to do. 

If you are just sharing the same router but not actually networked or sharing files, I believe you should be safe.