Sukishite Sadist (1996)

Blurb:
There’s a rumor that high school freshman Aki is involved with Masanari, a candidate for student body president next term. Aki thinks of Masanari just as her best friend, but to make sure of the matter, Masanari’s younger twin brother Naoya appears! Aki begins to be seduced by Naoya’s forceful manner...

Then one day, Masanari collapses. When Aki goes to wish him well, she’s relieved to see that he’s okay. While she’s there, though, Naoya asks her out on a date, and he’s being much gentler with her than usual. Aki musters up her courage and tries to tell Naoya how she feels, but not only does Naoya do an about-face and command Aki to date Masanari instead, but another girl confesses love to Naoya right in front of her!? What’s Aki going to do...??

Source: www.anisearch.com/manga/4639