So to answer the first question. no. Yoriichi is not Tanjiro's dad, that would be Tanjuro. And Yoriichi is not related to the Kamado clan at all.
Many Kimetsu no Yaiba fans deduce that Yoriichi is Tanjiro's father, but that isn't the case. However, Yoriichi is responsible for teaching Tanjiro's ancestors the Hinokami Kagura, which has been passed down for generations.
Tanjiro's connection to Yoriichi is not through blood relation, but through their similar appearances, which is due to the divine nature of Sun Breathing. (Also Read: Did Demon Slayer hire a Bollywood director for Season 3? Fans speculate!)
Tanjuro Kamado was the name of Tanjiro Kamado's father. He passed away before the event of the main story of the Demon Slayer.
Demon Slayer's season 3's latest episode has left fans on edge with Tanjiro's discovery of Yoriichi Tsugikuni's Type Zero sword.
Tanjiro's ancestor was a good friend of Yoriichi and arguably was one of his only true friends. Therefore Yoriichi gave the earrings as a gift, and since then they have been passed down the generations until they reached Tanjiro.
No, they are not related.
I'd even go as far as to say that he could contend with Muzan, possibly beat him if he was health. Even by the end of the series, Tanjiro admits that he's nowhere near as proficient as his dad was with the Breath of the Sun but still managed to keep a weakened Muzan at bay.
Tanjiro and his father look extremely similar to each other. Fans think that Tanjiro was related to the legendary swordsman since he had the same earrings, and their appearances are similar as well. However, Tanjiro is not a descendant of Yoriichi Tsukiguni.
That man, he wasn't Tanjiro's father. He was Yoriichi, the first and legendary Demon Slayer, who managed to defeat Muzan and almost kill him. That's him. Tanjiro's father wasn't a Demon Slayer, he had a special dance only the Kamado family knew.
What happened to Yoriichi in Demon Slayer? - Quora. Yoriichi Tsugikuni died of old age in his final confrontation with his older twin brother Michikatsu Tsugikuni aka Kokushibo.
Conclusion: Kokushibo, the upper moon rank 1 demon and Yoriichi are not Tanjiros' father and neither are Tanjiro's relatives. However, Kokushibo and Yoriichi are siblings and respect each other a lot despite Kokushibo hating the entirety of his brothers' being.
He was a man who received no formal training in combat and managed to take down a bear with a short axe. To sum things up, Tanjuro was one of the strongest characters aside from Yoriichi, given that he was able to access the Selfless State for hours on end.
It was because after being shown his overwhelming power, Muzan avoided him as best as he could. He didn't have another chance to kill him.
Tanjiro certainly did a lot of good with his Water Breathing techniques, but his ability to use Sun Breathing both gives him an edge in battle and allows him to stand out among the incredibly powerful Hashira he fights alongside.
Those who have read the manga know that Tanjiro's powers grow exponentially towards the end of the series. His skill and overall combat abilities could be compared to that of a Hashira's. That being said, there is absolutely no chance for Tanjiro Kamado to be able to beat Kibutsuji Muzan.
Moreover, Tanjiro's appearance somewhat resembled Tsugikuni. So the only reason Muzan is afraid of Tanjiro is because of the Hanafuda earrings he possesses, which acted as a reminder of his downfall.
The warrior Muzan feared was a swordsman named Yoriichi Tsugikuni. Yoriichi was the only slayer before Tanjiro who came close to killing Muzan, and the flashback of their fight occurred in Chapter 187.
Nezuko is at least stronger than Daki, meaning that if Muzan could bring her back under his control, she would certainly qualify for the most prestigious title a demon can hold: a member of the Twelve Kizuki. Thanks to Nezuko's remarkable strength, she would have to be an Upper-Moon.
He is the Mist Hashira and is considered one of the most talented swordsmen in the organization. He is younger than Tanjiro, and despite his age, people respect him for his swordsmanship.
Gyomei Himejima
Gyomei Himejima, the Stone Hashira, is the strongest Hashira in the Demon Slayer corps in terms of strength and swordsmanship. He is the only person who uses spiked flails and an axe instead of standard swords to kill demons.
The only reason why Muzan hates Doma is because of his insincerity and lack of emotions. This might sound odd, expecting emotions from a demon, but even a demon has to retain such tendencies to keep things in order.
Akaza was also one of Muzan's favorite demons out of all his subordinates. The reason behind this is Akaza's extreme loyalty to his master. The Upper Rank Three demon was always sent away on special missions, something the Upper Rank 1 and 2 were often denied of.
Muzan Kibutsuji has done some terrible things in Demon Slayer, but his slaughter of Tanjiro's family is one of the most mysterious.