Both manga are about two pretty hot brothers, opposites in both looks and personality, who are in love with their extremely plain short-haired sister.
Sonnan Ja Ne Yo is a shoujo, while Brothers is a comedy-centered shounen.
Both are comedy/romance series about two attractive and popular brothers who are both head over heels for their only sister -- domestic and romantic hijinks ensue. Sonnan ja nee yo has the added twist of one of them not being related to her, with the looming question being which of them it is, whereas in Brothers they're all related but their troll of a father keeps suggesting they might not be.
Romance between people living under the same roof in a complicated family situation. Marmalade Boy is between step-siblings in a silly two-family setup, whereas Sonnan ja nee yo has siblings trying to figure out how they're actually related and whether one of them is adopted. Sweet love stories and cute humour ensue!
Both are about a highly likeable protagonist who is considered the one plain-looking member of an otherwise gorgeous family, and though Sonnan ja nee yo gradually turns romantic, both are pretty darn funny.
Both deal with the development and obstacles within brother-sister relatioships. Cherry Juice tend to be more light-hearted while Sonnanja Neyo (Sonan Jyanaiyo) sometimes hold more serious thought toward the incest theme.
Both mangas deal with the development and obstacles of taboo relationships. In Ai Hime, it's between uncle and niece; while in Sonnanja Neyo (aka Sonan Jyanaiyo) it's between two brothers and their sister.
the protagonist of both is a seemingly ordinary and plain girl. she struggles with her friends and love interests in discovering who they are based on the past or their parentage. both mangas have a plot set in contemporary times and a subplot within the past that drastically affects the fate of each character