This is the 7812th most frequent Turkish word.
kardeşi
The Turkish word "kardeşi" means "his/her/its sibling."
Here, 'kardeşi' refers to Ahmet's sibling, indicating possession using the possessive suffix 'si'.
Ahmet'in kardeşi futbol oynuyor.
Ahmet's sibling is playing football.
In this sentence, 'kardeşi' denotes 'his/her sibling', using the third-person singular possessive suffix 'si' indicating ownership.
Kardeşi çok güzel bir hediye aldı.
His/her sibling received a very nice gift.
'Kardeşiyle' incorporates 'kardeşi' to mean 'with his/her sibling', here combined with the instrumental case '-yle' for 'with'.