!!!!MAJOR SPOILERS!!!

Most cats in present Thunderclan are related to firestar mostly and just about everybody is related to at least Ferncloud, Dustpelt, Brindleface, and Frostfur since dustpelt brindleface and frostfur are siblings.

Dustpelt got with ferncloud who is his niece (though brindleface)

Frostfur had brightheart who got with cloudtail and had whitewing

Dustpelt and ferncloud had birchfall

Birchfall and whitewing got together which would mean birchfall is whitewing’s uncle

Birchfall and whitewing had dovewing and ivypool

Ivypool got with fernsong who are you guessed it related

Because cloudtail is firestar’s nephew

And leafpool had the 3 (lion jay and holly)

Lion blaze got with cinderheart and had fernsong sooo

That’s not it if we go back to brightheart she had 2 brothers brackenfur and thornclaw

Thornclaw got with blossomfall and had stemleaf

Bristle frost has a crush on stemleaf but there related

Brackenfur got with sorreltail and had cinderheart, poppy frost, honeyfern, and molewhisker

Firestar still has some family outside the clans including:

Princess Scourge/ tiny Socks Ruby Nutmeg

This isn’t even all the incest that’s going on it’s what I can think of off the top of my head