This is the 7814th most frequent Turkish word.
kardeşleriyle
"With his/her/their siblings."
In this sentence, 'kardeşleriyle' is used to indicate the subject Ahmet accompanied by his siblings in the action of going to the park.
Ahmet kardeşleriyle parka gitti.
Ahmet went to the park with his siblings.
Here, 'kardeşleriyle' describes the entity Ayşe interacts with during her recreational activity while on vacation.
Ayşe tatilde sık sık kardeşleriyle oynar.
Ayşe often plays with her siblings during the vacation.
This usage of 'kardeşleriyle' emphasizes the collaborative action performed by Mehmet and his siblings.