If it's just your sister and not her friends (unless they're at your house), it's actually mostly easy.|
You need an internet connection for this, I have no idea how to do this on LAN connections without WIFI.
First, you'll need your private IP. If you are using Windows 8, Windows 8.1, or Windows 10, Open Task Manager. Go to the Performance Tab, and select either Ethernet or WIFI. Copy down the IP under IPv4.
If you are on Windows 7 or Vista, Right Click the Internet Icon and press Open Network and Sharing Center. Click Local Area Connection (or your wifi driver if you use wireless instead of ethernet), Details, and copy down IPv4 Address. Disreguard the rest.
If you are using windows XP or below, or you're running Novetus through Wine, you're on your own to find this out.
Once you have your private IP, write it down or remember it.
REMEMBER: IGNORE THE IP NOVETUS GIVES YOU, THIS IS YOUR PUBLIC IP, AND IS ONLY USED IF PEOPLE OUTSIDE YOUR HOUSE ARE JOINING!
Open Novetus and Select a Map, then go to the Host Tab.
The Start Server you select does not matter, but no graphics is faster. The port it is right now is fine, but you can pick your own if you know what you're doing.
Now, head into your sister's computer. (Or, if they don't let you on it, give them these instructions.)
1. Make Sure Novetus is Installed.
2. If it isn't, install it. (or ask them to install it.) If it is, Go to Step 3.
3. (You, or tell them to) Run Start Novetus.bat and Go to the Join Tab.
4. Set the Port to what you set earlier. (or tell them port should be X, X being the port you set.)
5. Set the IP to the Private IP You wrote down/kept in your head earlier. (or tell them to set the IP Address to the one you wrote down.)
5.5: If you haven't, get them to set the client to the client you selected, or they'll be unable to join.
6. Click Join Server (or tell them to)
If you did it correctly, they should be in. If you recieve an ID 15 or an invalid IP Address, you did it wrong.
Now, if you want to join yourself, go to Join Tab, set the PORT to the one you used to host, set the IP to "localhost" (localhost only works on the hoster's computer, so be careful!), and click Join Server.
If other people want to join, and they are not connect to your house's wifi, they will need your public IP. This also will include having to port forward or use UPNP, but you should save that for when you decide to host with people outside your house.