I’m sorry, I’m a bit lost What is the relationship between yuma and the old lady :) ...

something like that

the grandma and her husband both have B-blood types so they wouldn't be able to have an A-type child so papa ibuki realized yuma also isn't blood related and grandma confirmed with him that yuma is indeed adopted

Thanks for your comment. I got confused on that part. Now I understand ٩(๑❛ᴗ❛๑)۶