Ok but, I hate it so much when people say that Tanjirou and Muichirou are related. Like since when were they?? Some say that it's due to Tanjirou having the same exact eyes as Muichirou's dad, but that's not any proof? Yes, Tanjirou's eyes are similiar to Muichirou's dad's eyes, but it still wouldn't mean that they're related.
Some also say that Tanjirou is Yoriichi's descedant and since Muichirou is Kokushibo's descedant they're related. but the thing is that Yoriichi's only unborn child died when his wife also died. One fact known about Yoriichi is that he's pretty much the best husband, infact he held his wife's, Uta's, corpse for TEN days straight because he was so owerwhelmed with grief, so it wouldn't just fit Yoriichi's character to remarry. It's also pretty obvious that he thinks that he doesn't deserve to be happy again since he failed to kill Muzan and protect his loved ones.
One fact some people seem to ignore is that Yoriichi knew Tanjirou's ancestors. I think some people who say that Tanjirou is related to Yoriichi thinks that he was related to them. But Yoriichi actually saved the Kamados from a demon and became good friends, which is why is earrings and the sun breathing was passed down in the Kamado family.
And even if Tanjirou would be Yoriichi's descedant(which is impossible), it's been 400 years, I'm pretty confident that someone is related to their lover if looking 400 years back-