The noun (Substantiv/Nomen) Geschwister is feminine in German. Therefore, the correct definite article in the nominative singular is die. So, you say: die Geschwister.
(siblings)
Nouns ending with -nis and -sal: About 70% are neuter, often abstract or collective
Examples:
Mehrere Kinder derselben Eltern.
Two or more people who have the same parents.
Ich habe zwei Geschwister, einen Bruder und eine Schwester.
I have two siblings, a brother and a sister.