Can a PC on a router see data sent to a second PC on the same router?

If it is a switch (which most routers are) you can only see broadcast data. If the router/switch is a managed one (not a home one) then an admin can mirror the port (inspect packets) but all home routers are not managed.

A home user can rewire a sniffer tap or hub and that would allow it to be packet captured but that would require rewiring it.

So no, because it is a switch packets cannot be captured. The more likely access is actually doing some attack PC to PC because you are in the same network.

Not with a home router connected by Ethernet. The switch in the router will only transfer data to the designated target. For example, between a computer and the Internet or between two computers connected to the router. For Ethernet connections to a home router, spying on the communications of another computer connected to the router is not possible.

For WiFi connections, the situation is different. WiFi is equivalent to using a hub, and all wireless computers can potentially spy on the communications for other computers on the same WiFi network.

I hope this helps.

The router will echo all traffic with a local destination to all ports simultaneously. This behavior is by design. So yes, this means that all computers connected to any of the LAN ports of your router will 'see' the traffic destined to other PCs. If you want to see this traffic, install a network analyzer like Wireshark.

If the PC is authorized to monitor the router or if you have certain LAN software

The data passes through so yeah

I'd like to add I'm not a fan of motorola nowadays

Yes, if. **If** one of the PCs is running some network sniffing software and its being used by an experienced network admin.

Otherwise, No.

Yes, they can see the data that they are sending to each other.

Yes they can you need to use wireshark

http://sh.st/IFVo8