We have seen so many fans who wondered about Ash's father but there also some fans who wondered about Ash's long lost sibling. Well, I'm using "sibling" instead of "brother" or "sister" because it's unknown if his sibling was a boy or a girl. Some fans thought that Ash's sibling was a boy and some other fans thought that his sibling was a girl. I have heard a theory from some fans that Ritchie was Ash's long lost brother (This is one of the theories that I like but I still think that Ritchie have different parents). This theory sounds more interesting than the "Red was Ash's lost brother" theory in my opinion (
Yes, I don't like this theory!). I don't think that Ash and Red are related because they both live in an alternate universe (Ash lives in the anime universe and Red lives in the game universe). Of course, his sibling was never mentioned in any episodes but it would be great if they made an episode about him/her but sadly it will never happen.