Is Kokushibo Related to Tanjiro

Who is Kokushibo?

Kokushibo is one of the oldest demons besides Kibutsuji Muzan that the Audience knows. Kokushibo is well-known/famous among Fandoms for his demonic look and overwhelming Aura. 

Kokushibo/Upper Moon One

His First appearance was in Chapter 99 of Kimetsu no Yaiba Manga, during the meeting of the Upper Ranks. His age is more than 400 years. Furthermore in Manga, When Muzan was referring to Kokushibo, he referred to him as his Partner, not as a servant. When he became a demon, he quickly gained the title of Upper-Rank One due to his overwhelming Power. And was challenged multiple times for the Upper-Rank place but never lost.  

Who was he as a Human?

His name when he was a human was Michikatsu Tsugikuni, he was born during the Sengoku era and was the twin brother of the legendary Demon Slayer Yoriichi Tsugikuni, the inventor of Sun Breathing. As a Human, he was a respected Demon Slayer but was jealous of his brother due to him being inferior to Yoriichi's Strength. Furthermore, he was unable to perform Sun breathing, so he developed his own breathing style and named it Moon Breathing. Later he Unlocked his Demon Slayer Mark(Unlocks in Die or live Situation), but even though he was not on par with his twin Brother, and was anguished thinking about how he would die reaching at the age of 25. His fear of death led him to become a demon, thus becoming immortal. He abandoned everything related to him, his wife, his children, and everything and became the subordinate of Muzan Kibutsuji.

Is he related to Tanjiro?

Is he related to Tanjiro? No, he does not have any direct relation with Tanjiro. The only connection between Tanjiro and Tsugikuni Family is through his twin brother Yoriichi's Sun Breathing Technique, other than that they have no direct relation.