I just read this one after putting it off for a while:
https://www.mangago.zone/read-manga/falling_to_where_i_belong/
Just to clarify (since it also pushed me away from reading it), MC and ML are not related/step brothers.
If you haven't read it, Side Top is also really good (not the actual title, but it has side top in it). It's about a person who gets thrown into the novel as the "side top" of a bl novel. In the original he dies early on. There's 2 adorable kids (ML's brothers) in it too. Nearing the latest chapters you'll ask if the fluff in the beginning was real or not because it did a 180 from fluff to angst.
https://www.mangago.zone/read-manga/life_senjou_no_bokura/
it's so cute but also made me cry. It's short but will get anyone right into the feels. The angst feels real.
can anyone recommend me a bl manga where theres like lots of agnst but also fluff like i need some gut wrenching stories. most preferrably i want mangas but manhwas work just fine too! :D