As a little sister, yes it is permissable to crush on your siblings lover.

[kat8cha]

Miyuki isn't sure of the exact time she starts crushing on Tachibana Kippei.

Miyuki is, however, relatively sure that her crush began the night her brother brought Tachibana home and introduced him as 'the guy who'll help me win nationals'. Tachibana had looked gruff and angry and had elbowed Senri in the ribs. Miyuki had thought that anyone who could push her brother around was just a little bit cool, and thought Tachibana was even cooler when they stared talking about tennis over dinner. The Chitose household infrequently met for meals, but her brother was always trying to make time for Miyuki. Tachibana came over for dinner several times after that and each visit began with Miyuki feeling unsure and acting very brash. Tachibana would ruffle her hair and barely give her a passing glance as he followed Senri into the house.

 

Miyuki can pinpoint the exact time that she realized her brother was gay. They had both come down with a fever, Miyuki had first and then Senri had caught it while taking care of her. Tachibana had come over his worried footsteps causing Miyuki's fever headache to throb in time. He had entered Senri's room and there had been muffled talking. Feeling adventurous Miyuki had slipped out of her bed and padded towards her brother's room.

 

Senri's door was cracked just the smallest bit, a sliver of light clear in the dark hallway. "Your sister's right down the hall." Tachibana grunted, and Miyuki held onto the wall as she peered into her brother's room. 'Are they wrestling?' she had wondered when she saw her brother pinning his friend down.

 

"I'm sick Kippei. Indulge me." And then Senri had kissed Tachibana and Miyuki had to flee to her room before she did something to call attention to herself. She sat on her bed, silent tears sliding down her cheeks because of her first heartbreak.

 

Miyuki hid whenever Tachibana came over after that. Now that she knew Miyuki could not help but see signs of just how into each other her brother and his boyfriend were, and every time Tachibana's eyes would darken while watching Senri made Miyuki's heart drop into her stomach. She thought about telling them that she knew, and Miyuki thought about telling their parents, but she couldn't do that to her brother no matter how upset she was.

 

Miyuki knows the minute she started hating Tachibana Kippei. Miyuki started hating Tachibana the minute Senri opened one eye, the other covered by layers of gauze, and looked around in confusion.

 

"Where's Kippei?" Miyuki, clinging to his hand felt anger. Outside of the room her mother was talking with a doctor about her brother's chances of ever using his eye again and Senri asked about Kippei. She opened her mouth to spew hateful, angry, vile words about Tachibana.

 

And stopped. Miyuki knew that Senri would listen to her if she did not like Tachibana, in fact he might even break up with Tachibana over it, and for the same reason Senri would have done that for her Miyuki held back her sharp tongue. She might hate Tachibana, but she loved her brother and couldn't hurt him.