"My sibling is Greek."

[Mirage20]

Benim kardeşim Yunanlı should be acceptable as well.

[AlexinNotTurkey]

Yunan means "Greek." Yunanistanlı means "from Greece." :) Yunanlı is not a word

[scuzbucket]

Gayet kullanıyoruz biz paşam

[Selcen_Ozturk]

"yunanlı" is an incorrect usage