in my minds the horsemen aren't related because I see them as separate "monsters" all having their own unique abilities, however they are treated like a sort of extended honorary family since they are a close nit group and have known each other for most of their lives.
I love his design! Marnie would like to have him as a friend. Made her a LONG time ago so it has it's flaws but I think they could be good friends. If you have interest you can see her here: [link] Great character design. Really like it.