Did the Greek god Pan have any siblings?

Yes, as any son of Hermes would be a brother of Pan. Immortal divine sons of Hermes included Hermaphroditos and Priapos (mother being Aphrodite) Pherespondos, Lykos, and Pronomos his Satyr sons by Iphthime.

Eleusis (which is sometimes female, sometimes male) by Daeira.

And of unknown mother : his daughters Angelia and Palaistra.