They're littermates (what you said in the tiny print)
And did you ever notice that Tigerstar and Firestar are brother-in-laws? Think about it... Firestars daughter Squirrelflight is mates with Tigerstar's son Brambleclaw, and in my opinion, having a mate is like being married so... That would also mean that Graystripe is related to Firestar!
Greystripes daughter, Feathertail, mates with crowfeather, then Crowfeather mates with Leafpool, Firestar's daughter... EVen though they're related weirdly, they still are XD Also, Silverstream, Greystripe's mate, is Crookedstar's daughter, meaning Crookedstar's related to them too, AND (omg lol) Crookedstar's brother, Oakheart, is mated with Bluestar (well was, but it still counts XD) soooooo Bluestar is related to all of them TOO!!! And that means that if Scourge is like Firestars brother, and Tigerstar is HIS brother-in-law, then SCOURGE KILLED HIS BROTHER!
That was confusing... sorry... XD
Wow I wrote alot...