Someone sniffing my traffic

I do a lot of work at home and have some (although slight) suspicion that my traffic from my PC to the outside world via a modem may be being monitored (we have a resident expert who might be inclined). I share the link with someone else by pulling out a USB connection to the router and plugging the other PC into the single USB socket on the router. The router has a RJ45 but we are not using it.

1. Would someone be using a packet sniffer and how?

2. What other methods may be available to them e.g. installed software on one of the local devices?

3. Can I do something like secure my traffic to make it more difficult?

Of course I know that internet traffic can be analysed by asking for details from the supplying ISP but this is unlikely be available to this case.

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