The wireless router should have a WAN port and one or more LAN ports, The modem connects to the WAN port via the ethernet cable, your PC connects to one of the LAN ports via ethernet and her laptop should use a wireless port, though you will need to read the instructions and set up the wireless secirity first. If you're gonna ask how to do that it's best if you post the exact make/model of the router and I'm sure that someone with the model will be along shortly and explain it all
Just for information when you connect the router power off all the equipment then first power on the modem, then the router, and finally the PC
