I’m a green card holder who travels fairly frequently and have faced no issues on re-entry, and most don’t. Waiting periods for naturalization depend on city the person is in, and people don’t get deported for arbitrary things, only for particular crimes.not sure why all this scare mongering here. The OP and family are presumably aware of the residence requirements hence they are all returning. Not sure this is the forum to question their financial decisions, and as green card holders any of them can get jobs if they find themselves needing more cash. Green card holders are eligible for federal college financing, work study programs etc too.
Back to the actual topic of high school: one thing to raise is to be sure your sister brings a copy of the vaccination worksheet done for the green card medical as she will need that for school enrollment (it’s much easier understood than any national vaccination card). A transfer report from her existing school might be useful too, international schools are used to doing these.