i dont think theyre siblings. my theory goes like. henri at a very young age met iruma's mom right? another human girl (this is ameri's mom) got lost in the demon world and becamse friends with iruma's mom. ameri's mom, iruma's mom and henri got close (kinda like azaz-clara-iruma friendship). ameri's mom got pregnant and gave birth to ameri (either she passed away or she was able to return to the human world) while iruma's mom was able to return to the human world. this can explain why ameri has a collection of mangas (assuming the mangaka is iruma's mom) bc maybe she sent them to ameri as gifts

So she is the author of Hatsukoi memory , huh? That's why Henri has all the issues of the manga , and it's kindda sad to know that if the author of Hatsukoi is Henry's wife then Iruma has meet Ameri's mother but Amerri probaly will never be able to see her.