I imagine the most significant of your mentioned connections is that they are created by the same developer - Success.
Having similar themes and a similar art style is definitely enough to warrant the title of sister games.
As a Western example, Bastion and Transistor are also sister games - They share no common story, but are similar in artistic style, development company, musician and artist.
The term sister in sister-game is broad and stems from sister cities & sister companies. The latter of which shares no more than the same parent company. Sister cities are broad, officially approved, long-term partnerships between two communities - but they do not have to be culturally alike or share many traits.
The requirements for shows to have an implicit connection is low, and can be justified by being in the same broad universe, parallel universe, etc. It's mainly a production choice.