This is the 3366th most frequent Turkish word.
kardeşlerin
"Your siblings" or "your brothers and sisters."
In this context, 'kardeşlerin' refers to 'your siblings', marking possession with the suffix '-lerin'.
Kardeşlerin okulda nasıl başarı gösteriyor?
How are your siblings achieving at school?
Here 'kardeşlerin' means 'your siblings' as the subject of the sentence indicating a group.
Kardeşlerin arasında anlaşmazlık var mı?
Is there a disagreement among your siblings?
In this usage, 'kardeşlerin' identifies the possessive 'your siblings' as performing the action of coming.