It's both. They are the predecessors of the main cast and are also reincarnations of the main cast if I'm reading this correctly. However, what has me confused is that would mean that nezuko's granddaughter and tanjiro's grandson are dating...which isn't the most unsettling thing in the world but is kinda awk as they seem to be unknowingly be dating each other...not knowing that they are second cousins. (= _ =')
They mention Zenitsu is their great grandfather, so by this generation, they're only distant relatives and its kind of normal in Japan to date a distant cousin or something
I also think it's both, relatives and reincarnations. Obviously those who died at the demon war didn't get to have a family and the survivors continued living and here we are seeing their predecessors. Just like Mitsuri and Obanai promised that when they are reborn they would find eachother and get together. And I think that the reincarnations were perfect, Gyomei as a kindergarten teacher was so cute, Mitsuri and Obanai running a restaurant was great, ridiculously large portions for Mitsuri used to have a huge appetite :'D

Im honestly quite confused, is it their relatives or them reincarnated with different appearances and names (also different personalities)? The hashiras make small appearances and they (main characters of this) look a lot like the original cast, also along with the minor characters (Yushiro, Sabito, and Makomo). We can also see Mitsuri and Obanai (or at least their relatives) running a bakery(?) together and they look like a couple like how Mitsuri and Obanai promised to be once they are reborn. But I’m guessing from the picture, it is framed because it is a picture of the main character’s “ancestors”. Again, I’m still confused. If anyone knows the full story (or think), mind replying and explaining it to me? (づ ̄ ³ ̄)づ