If you're searching for manga/manhwa with similar themes (older sister figure + secret tutoring + romance/comedy), consider:
A long pause. The longest yet. Then she turned her head slightly, just enough to show her profile. The soft light from the window caught the edge of her glasses. “No,” she said quietly. “Only you. That’s one of our secrets, isn’t it?” katekyo kireina onesan to himitsu no lessons full
He sat up quickly, his face warm. “Sorry.” If you're searching for manga/manhwa with similar themes