Timoteo, Vongola Ninth
Timoteo is the ninth boss of the Vongola family, his mother was the eighth boss, and his great grandfather was the seventh. Much of the Vongola power is drawn from bloodline. Despite knowing that Xanxus was not his son, he took the child in and raised him as his own. He wrote an entry in a journal stating that although he knew, he could not leave the child on the streets. Due to the Vongola power being based in bloodlines, Xanxus has no actual tie to the Vongola once he finds out that he is in no way related to Timoteo, this also meant that he could never be the tenth boss. In this case, Timoteo's kindness is what prompts Xanxus' rage. This betrayal is what leads Xanxus to try to kill him, what leads him to kill his brothers, and later what leads him to try and kill Tsuna. Despite all of this, I do believe that Timoteo spoiled Xanxus rotten and that's one reason that he turned out the way he did. Raised as a Vongola despite not being of Vongola Blood still made Xanxus feel as though he were superior to everyone.
Unnamed Mother
Xanxus and his mother lived in poverty. She convinced herself that her son was the illegitimate son of the Ninth Vongola boss after witnessing Xanxus' flames. She believed this with every inch of her being and often told Xanxus that his father was the Ninth. After presenting the child to Timoteo, Xanxus was adopted into the Vongola family. Nothing is said of Xanxus' mother again, but I believe that he genuinely believed her because he is shocked and feels betrayed by the knowledge of his adoption and the idea that he is in no way related to the Ninth Boss.
Brothers: Enrico, Massimo, Frederico
Xanxus seems to have a very love hate relationship (as in he loves to hate them) with his three adoptive brothers, Enrico, Massimo, and Frederico. Not much is said about them except for the fact that as tenth boss candidates they were killed by another candidate. Since Reborn delivers that news to Tsuna, it is safe to assume that Xanxus is the one that killed the other candidates. I also believe this is an easy assumption to make because he spends so much time trying to kill Tsuna. Enrico is most likely the oldest brother and for that reason most likely to become the next tenth boss. He died in a gunfight. Massimo was the next in line after Enrico, he drowned. Frederico was the favored son of the Ninth Boss and for that reason, I believe that Xanxus and the Varia made him suffer and that's why all that remains are his bones. Xanxus strikes me as the youngest and after finding out that he was adopted, I believe he took out part of his anger and frustrations on his brothers, particularly Frederico who had their father's vote for candidacy.