In BOKU NO FUTATSU NO TSUBASA was Hiromi adopted? It was said Kaneko (Hiromi's mother) was actually Mako's biological mother. It was also said Hiromi and Mako are siblings but they are not blood related. If she was adopted how did she come into the picture?

She is adopted (I remember something about her being only a few months older or something). How she became adopted I don't remember.

Post topics related to a specific series under its own forum next time.

Any idea roughly where it was revealed she was adopted? Somewhere in the middle of the series etc. It was revealed Hiromi was three months older near the end of the series chapter or two after they find out Kaneko is Mako's real mother.

Hiromi is Mako's biological cousin, but since the woman Hiromi calls "mom" (adopted her after her own parents' deaths, no details given) is revealed to be Mako's birth mother, that makes her both adopted sister AND blood cousin.

Kinda funky how the daughter thinks her mom is an aunt who sorta-adopted her, and the niece calls the aunt who adopted her "mom"...

The adoption of Hiromi was mentioned just the once, during some rooftop conversation where Hiromi is reassuring Mako, probably about being accepted by someone as usual, or maybe about Mako's childhood as Grandpa's ward. Something like "You know, I'm not actually mom's daughter, she took me in when my parents died three (?) years ago."

The BIOLOGICAL COUSINS conclusion is based on:
a) Hiromi calling grandpa grandpa,
b) Hiromi viewing Mako as a cousin from back when they were little kids, before she was adopted,
c) Hiromi and Mako knowing each other at all
d) Mako actually being Grandpa's grandkid, just from a different daughter than she was told

...so, apparently, the dead daughter's kid calling the living daughter "mom" is Hiromi, and the living daughter's actual kid Makoto got fed a lie about her parentage based off her cousin Hiromi's true story. Seems their life stories got 'switched' for convenience's sake, probably so that the lies don't accidentally come to light in some situation like "wait grandpa, how can I be the grandkid with the dead parents if you never had a kid who died??"

Hiromi's past appears planned but never drawn due to rushed ending... Seems to me like the three-month difference was named to assure fans that they were never intended to be blood sisters in any cut subplots (prob since kissing cousins seems to be 100% acceptable in Japan... then again, if you go by what you see in manga, so are teacher-student raep and having sexy-time with your mommy, so, YMMV).

Hiromi is adopted, and they are not related by blood at all. I don't know where they say it, but I'm searching... 😀 😀

Found it. Chapter 8, page 24: "I'm not connected to my mother through blood."

"Thus is our treaty written, thus is agreement made
Thought is the arrow of time, memory never fades
What was asked is given, the price is paid"

indeed she's adopted

near ending was revealed that hiromi is an adopted kid as replacement of mako after mako's birth in order to cover the truth about mako's real identity as half human.
also to protect mako from to be examined by labs and stuff.
other purpose is to console mako's mother's heart after gave birth