step brother?
http://www.mangago.zone/read-manga/sexual_harassment_brothers/
http://www.mangago.zone/read-manga/home_sweet_home_bori/
http://www.mangago.zone/read-manga/egoism_tim_alchem/?czsV
http://www.mangago.zone/read-manga/aniki_to_isshoja_nemurenai/
http://www.mangago.zone/read-manga/kyoudai_no_rule/
http://www.mangago.zone/read-manga/netsuai_naono_bohra/
http://www.mangago.zone/read-manga/bokura_no_negai/
that is not adopted, but you must read it ˘◡˘
Yes!! I strongly recommend Bokura no negai!! The story was so heartwarming and moving.
Oh, but please read Mankai Darling before you read this one. The main characters in Mankai Darling appears in Bokura no negai and if you know them beforehand, it makes the hole reading experience even better! Mankai Darling is not about love between family members but it’s a short story that does not take too long to read through!

I want to read some manga or webtoons that the plot is all about a love story between in a family son and an adopted one which has no blood relationship at all, Thank you ^_^