Roommate Network
More information would be helpful here. I'm going to assume that the home network is just a wireless router that you all share? If so, there are a couple things you can do. I'm assuming a normal set up and there isn't anything strange like the firewall being disabled or etc.
Make sure your computer is fully updated and backed up. Download one of many free antivirus scans. Your major risk is if their computer gets affected and is then attacking you directly so make sure you are protected.
Check the home router to see if it has a setting called "network isolation". This will be by far the best protection you can enable if you have that option.
Know that the other computers on your network aren't a threat to you unless they get infected, or one of your roommates is being malicious.
Set up multiple networks. Some routers have the ability to set up separated networks. That can be a guest/main, or 2.4 vs 5 G, or just multiple SSIDs. This will help by creating some network segmentation away from your roommates.
Talk to your roomates. If you don't feel good directly confronting the sketchy behavior, you can still ask them what can be done to make sure your network is as secure as possible.
First, get your own router between your computer and their network. The NAT function
*(Yes I know, NAT is not officially a security feature)*
will help isolate you from unsolicited connections.
Second, use a VPN (roughly $40/year) for all your connections. This will prevent them from seeing your activity.
Third, if your roommates attract the notice of law enforcement, LE will respond based upon the IP address. Have an attorney number memorized, because they will take your phone, along with everything else.