Both animes are an 2019 winter season's adaptation of the romantic harem manga from Shonen Magazine, which are dedicated to MC’s relationship with several girls who are sisters. They even received a joint cover a few days before the premiere. At the same time, these shows are opposite in their portrayal of romance, while Go-Toubun is mostly quite subtle about the affections of girls, Kanojo openly depicts the romantic and sexual life of the characters. In addition, the first show is a lighthearted comedy, while the second is a somewhat dark melodrama.
They both maybe a harem, but then gain this is a reinvented harem where it's one guy to sisters and yes it's plural, except in Domekano, it was more of a Ménage à trois, while in 5-toubun no Hanayome it was a bit more complicated.