No, Firestar isn't related to Graypool.
His two daughters are Squirrelflight and leafpool (daughters of firestar and sandstorm). Firestar does not have a half-son but he has a half-brother who is scourge (because they have th same father jake).
Graypool's father is Reedfeather of Windclan
Firestar's daughter is Leafpool and his half-son is Brambleclaw. Leafpool is a medicine cat in ThunderClan and Brambleclaw becomes the leader of ThunderClan, taking the name Bramblestar. Both cats play significant roles throughout the series.
The only one Erin mentions is Princess, who's son is Cloudtail
Firestar's kin are: His sister, Princess His nephew, Cloudtail His two daughters, Squirrelflight and Leafpool His five grandchildren, Hollyleaf, Lionblaze, Jayfeather, His nephew's daughter, Whitewing His nephew's daughter's two kits, Dovepaw and Ivypaw His half brother Scrooge
Yes, because Pinestar's mother is Nutmeg and Jake(Firestar's father) is mates with her, they are quarter-brothers(Firestar is Nutmeg's son, whereas Tigerstar is Nutmeg's grandson). Cool, eh?
Firestar
Firestar's great-grandchild in the Omen of the Stars series of Warriors is Alderheart. He is the great-grandson of Firestar, through Firestar's granddaughter, Squirrelflight, and her son, Leafpool. Alderheart is a significant character in the Omen of the Stars series, serving as a medicine cat of ThunderClan.
They might be since Firestar's daughter, Squirrelflight, is Brambleclaw's mate.
He offered Firestar to sleep in his backyard for one night.
No . Firestar's kits are Leafkit/paw/pool, and Squirrelkit/paw/flight. Graystripe is Firestar's best friend and Silverstream was his mate.
this is an opinion to me it would be Firestar