Here's how:
Graystripe's mate is Silverstream, who's father is Crookedstar, who's brother is Oakheart, who's mate is Bluestar, who's sister is Snowfur, who's son is Whitestorm, who's daughter is Sorreltail, who's mate is Brackenfur, who's sister was Cinderpelt, who's sister was Brightheart, who's mate is Cloudtail, who's mother is Princess, who's brother is Firestar.