If you’re asking this question at all, I’d say you’re probably not ready for a minimal installation without a desktop environment but it’s totally possible.
I’d recommend just going with a normal distro and installing a desktop environment from the get-go as that will be the easiest way to get Linux set up.
That is neglect. Please call whatever authorities are relavent in your area (list of DHS numbers for the US: https://www.childwelfare.gov/state-child-abuse-and-neglect-reporting-numbers/) and they can help you.
I hope you and your sister find the support you need.
You can look up all your Steam games on protondb.com but I’m not sure how to check Epic Games compatibility beyond just installing the Heroic Games Launcher and trying them out.
I hope you have backups, man.