I have two computers connected to an ethernet/wifi dlink router:
Laptop: Connected via Wi-Fi
Desktop: Connected via ethernet
My understanding is that the desktop computer will never see any packets exchanged via Wi-Fi between the laptop and the router which means, I suppose, that the desktop computer cannot sniff any FTP passwords that I use to connect using the laptop (without SSL)
Is it possible to set up the router to send copies of Wi-Fi packets to the ethernet interface?
It highly depends on the (SOHO-) router and its internals if sniffing is possible. Usually Wi-Fi and ethernet ports are separated and you can't sniff Wi-Fi with an external ethernet device. Even if your router contains several ethernet interfaces and all of them are attached to an internal switch, packet capturing of the other ethernet ports is impossible.
Some routers contain an internal sniffer though. My router (AVM Fritz!Box 7490) provides a sniffing feature for diagnosis:
After choosing one or several of the different interfaces and hitting one or several start buttons I get a full capture in Wireshark format.
Your D'link router may contain a similar feature. Or a free firmware with this feature exists for your router.
A second option is a Wi-Fi sniffer attached to the desktop computer.
