Migawari de Endan ni Sanka Shita Gumai no Watashi, Ringoku no Ouji-sama ni Misomeraremashita

There is only one saint per generation, but Hestia and Astrea were born as twins. However, only the older sister, Hestia, can perform miracles, while the younger Astrea is looked down upon as incompetent. Her arrogant older sister forces her to take her place for marriage proposals, but the man she meets there is Silvert, the second prince of a neighboring country, and they end up getting engaged...!? (Source: Gaugau, translated)