Can you spy on someone's device if they share the same router. I suspect my homestay brother is hacking my device. What would you do? I told my mom and she is researching how routers work. Sadly mine (and with my mom's) comp skills etc are bad. This student is creeping me out. I told my mom that too

To a first approximation, if they're on the same router as you are, it's easy to spy on your network traffic if you don't use HTTPS (to an even rougher approximation, HTTPS is sites that have a padlock in your address bar). If you use HTTPS, you're theoretically protected from the simplest kind of packet sniffing, but in practice vulnerabilities exist (there was a recent one published at krackattacks.com).
Beyond that I guess the question is kind of insufficiently defined depending on what it means to be spying on or hacking a device. Based on the one security-minded friend I consulted, you should be more concerned about somebody physically accessing your devices, guessing weak/shoulder-surfed passwords or plugging in peripherals.
I am not a security professional and am kind of assuming a paranoid hypothetical worst case; use your best judgment.

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