18+ Moto Mahou Shoujo no Oneesan wa Koi wo Shiritai

Touri, a college student searching for his older sister whom he was separated from 10 years ago, discovers that his admired senior, Sara, is a former magical girl, and that his sister was once a magical girl as well. He continues his search for his sister with Sara's help, but it seems the magical girls who have become "older sisters" can only draw magical power from heart-throbbing romantic excitement...? The harem romance comedy featuring a former magical girl "older sister" and her "little brother" begins!!

(source: MangaDex)