If by "a thing" you mean, did they have children, yes: Despoinê and Arion were their offspring.
Demeter and Hestia were her sisters. She also had three brothers Zeus, Poseidon, and Hades.
demeter
Demeter asked Poseidon to help her find her daughter Persephone, who had been taken to the Underworld by Hades. Poseidon revealed that Hades was responsible and guided Demeter to him.
no
Poseidon had relationships with multiple women such asMadusa and Demeter
Poseidon pursued her, and when Demeter would have gotten away as a mare, Poseidon turned into a stallion and mated with her, the children resulting were Despoine and Areion -this after Persephone was taken.
Yes. Poseidon presented Demeter with several horses to impress her.
Demeter's siblings are Hera, Hestia, Hades, Poseidon, and Zeus
Medusa, Demeter and Amphitrite
Demeter and Hestia.
Zeus, Poseidon and Hades.
horse