Ryuujin-sama no Futago no Tsugai

Born into a branch family of the prestigious Asagiri family, which bears the role of the "Dragon God" protecting the imperial capital, Hanamura Tsubomi has been oppressed as an outcast. In contrast to her younger sister Saki, who is favored by their parents, Tsubomi's existence is not acknowledged by anyone. But one day, she meets Homare, the next head of the Asagiri family. The moment their fingertips touch, a glowing "Dragon Seal" appears!? Homare declares, "I will take you as my bride." However, due to the machinations of both families, her younger sister Saki also ends up living in the main house as a "bride candidate"...

(Source: Cmoa Comics, translated)