Cousins is kind of weird but 1.) I doubt the manga will ever get that serious about it and 2.) They weren't raised together for long enough for there to be any weird power dynamics or anything going on, so honestly whatever.
@hiddenz08 Lesbians are a separate issue, but the relationship between cousins is considered normal in Japan. In any case, I don’t think that this is some sort of romance, rather a typical fanservice in the spirit of any all-female sports titles. Of course, yuri bait is intentional, because the author has already painted yuri illustrations and it is still Manga Time Kirara, however such works rarely go beyond any teasing or simple fanservice.