From what Sena said, I wouldn't be surprised if her breakup with Makino's sister was over having to pretend to be a man in front of their family and/or her sister's shame in being unable to introduce Sena as a woman/girlfriend.
What was her name...? Reina. I'll try to remember it.
No doubt about it, all the conventions and the way it's set up this is definitely the case. Sena is definitely sad and fed up with Reina pretending when she'd rather be open about it.
I can vividly see Reina and Makino being completely flabbergasted that Sena would wanna be open about it to their parents. Like "Can you not tell how strict they are? Makino here was sent to a cramschool all vacation long just because mom found a card she didn't like in her room. We are fighting over who should take the bullet and take over their soul sucking business. We'll be disowned at the mere suggestion that we aren't straight!!, You're crazy Sena, crazy we tell you!"