The actual correct singular form of "[die (since plural)] Geschwister" is "das Geschwist" - a word that is rather obscure even for many native German speakers.
Correct.
> However, I wouldn't be surprised if there were a lot of exceptions to that rule.
Not to this one.
I never heard that used as a singular noun. Maybe it is a Swiss thing. If anything you could say 'das Geschwisterkind'.
1. (männliche wie weibliche) Kinder gleicher Eltern, nur im Plural. Beispiele: "die Geschwister sehen sich alle ähnlich", "ich habe drei Geschwister (wir sind vier Geschwister)"
2. einzelner Geschwisterteil. Gebrauch: Fachsprache; auch schweizerisch Beispiel: "das ältere Geschwister"
So, you are correct, the singular form is only used by the Swiss and as a technical term. So maybe the game shouldn't contain it at all? Or accept both "das" and "die", in case someone thinks it has to be plural?
The feminist crowd just perceives a common male word as gendered and a common female word to be not.