In the family tree drawing of Sirius black's house its been show that most of the slytherin families are inter-related with each other. Is there any reference that all the slyterin families, mostly the pure bloods are somehow heirs of Salazar Slytherin? That if their family tree is tracked back to Salazar Slytherin?
I think it was stated on second book, that Tom Riddle was the only living direct descendant of Salazar.
Quote from "Harry Potter and the Chamber of Secrets" :
There is no way to know this as I can't find anything saying this straight forward, but since intermarrying occurs then people are going to be related. I know that Dumbledore said in the Chamber of secret that Voldemort is the last descendant of Slytherin, but in defense of my statement, I think he meant that Voldemort is the last one to be directly from the Slytherin bloodline, as Harry is related to Voldemort and therefore related to Slytherin, but in a roundabout way.
I would hazard to say no, the Slytherin lineage can not be traced back to Salazar Slytherin through canon,alone. I would say that it is possible through mostly canon material, though,to trace back some of there lineages.
We know the Gaunt's are related to him for sure, and since many pure-blood families intermarry, then that would also indicate that they as well would be related to him. But since the whole of the wizard community that cares about "magiciness" or being pureblood intermarry within anyone that would mean that most of the wizards from any house have not only Slytherin blood but also some other blood(like Hufflepuff or the peverell's). Thus, while not proven it is obvious that a lot of the wizards in Slytherin are probably very distinctly related to Salazar Slytherin.
On another note we can see from the Harry Potter and the Voldemort situation that anybody can be related to anybody, as wizards interbreed.
Hary and Voldemort are related through the Peverells, this was talked about in Deathly Hallows. Also, it depends on how you see that people were created or otherwise, but are technically distinctly related.
Possibly the reason you mistaked all Slytherins for being related to Slytherin is because that they are chosen by the sorting hat. But the sorting hat doesn't just choose, because of blood it chooses because of your qualities and what you say to him( at least in Harry's case).
Also, from non-canon sources there are these pictures. If you search for "Slytherin family tree" you get a lot results.