You know, with Tsubame "only ever being in love with baseball," being refreshed by the sight of a cute girl, laughing at her brother being a virgin, and directly comparing how she should approach Kamei to how she'd approach a date, I think she might just be a lesbian who's already accepted it.
That might also explain her anxiety about asking Kamei out to another baseball date, both as worries over being accepted (though she hasn't even come out) and also due to her anxiety as just a baseball fan.
Also, her brother thinking she's in relationship with a guy when it's actually a woman is great. Granted, I am the type of person who enjoys the "friends think it's them being influenced by someone they view as a threat when it's actually a gay crush," type of misunderstanding. (i.e. thinking they're getting bullied when its a crush, thinking its them getting a boyfriend when they're actually being influenced by another woman)
The author and artist of this series is also the artist for Girl Meets Rock!, which is a series that has good LGBTQ+ rep, with one canon lesbian (as in she directly states she was attracted to a girl), one INCREDIBLY likely lesbian (nothing this girl does can be taken as straight), one likely lesbian (holds admiration for another girl that just so happens to put her in the same exact dynamic as the lesbian and her former crush), and an ace character (she talks about how she's never held romantic feelings for another and is frustrated that other people can't accept that).
One last thing: both Tsubame AND Kamei have gay little fangs. Very important.