as the title says! it's a manga with a historical fantasy type of setting so they're noble ladies of some sort. the plain-looking sister was betrothed to a guy who the prettier, sickly sister was also in love with or something.

random detail: the plain-looking sister likes to look at ants on the ground.

[spoiler] they get found out - i think. sickly sister's soul was betrothed in plain sister's body, as planned. but the guy liked the plain sister all along so he got married to the plain sister's soul in sickly sister's body instead. iirc the reason why she's 'sickly' is an abundance of magical powers or something. also she ate up her favourite cookies that the guy brought as test-bait. (sickly sister's soul declined the cookies to maintain her figure.)