Seinyuu no Miko Onee-chan Ruri ~Iyashi no Bonyuu de Nakadashi Seikatsu~

The lead character has a childhood friend who has become a shrine maiden.
One day, he is put to the test to marry her and receives a curse of death.
It turns out that only her milk has the power to cancel it.
And this is how his love life with her, healed by her breasts and milk.

[From English DLSite]