That was a mistake. Thanks - I've removed that now.
Is it right that the only difference between 'the brothers' and 'the siblings' is the gender of 'the'. E.g. 'οι αδέλφια' (the brothers) versus 'τα αδέλφια' (the siblings)?
No. The plural of ο αδελφός is οι αδελφοί (and that of η αδελφή is οι αδελφές).