In the 104th Trainee Squad they would always tease Armin.

They'd call him a child and also treated him like one.

They also did this to Krista.

They even made up parents for him.

Armin's parents were Mikasa and Eren and Krista was his sister.

Reiner and Annie were the Uncle and Aunt(this made Bert super jealous of Reiner).

Thomas was the cousin.