I think that Poseidon's siblings are Zeus and Hades.
The siblings of Poseidon were the children of Rhea and Cronus; his brothers are Zeus and Hades, his sisters Hera, Hestia, and Demeter.
the function of Poseidons temple was for Poseidons honor.
Rhea.
cronus
Cronus and Rhea
Triton
Neptune
Rhea.
a trident
He doesn't had friends
In myths, he does not have one.
you don't
Give it back to Poseidon.