I found for me that the "missing games" weren't actually games. I have my library set up into 2 Collections: Favorites and Uncategorized. You can see that there is a lot missing from Uncategorized:
Similar to the answer from @Joachim, if you go into the filter settings and select Unplayed, a new category called "Tools" will appear. ...
This might not fix it, but if anyone has this issue make sure you are allowing Java Area Runtime through the Windows Firewall.
If Minecraft is still running and you cannot stop it by normal means, go to CMD, and enter this:
taskkill /im javaw.exe /f /t
This ends Java Area Runtime. Sometimes tasks won't stop themselves when ...
The best way is Hamachi.
Download Hamachi on your PC, Register in the app. Then create a new Network (With any name and password). Tell the name and password to your sister, so that she can join. After she did it, she must copy your IP address (it is shown in Hamachi).
Then you just enter your world and open it to LAN. Tell your sister the PORT, which is ...