
Unfortunately, I don't know any manga or webcomics that fit that description, but there are a couple of web novels that do:
- The General’s Manor Young Concubine Survival Report (modern girl reincarnates in the body of a canon fodder villainess and decides to avoid a bad ending by pursuing the female lead instead of the male lead)
- Mistress, I Was Wrong (similar plot to Concubine Survival Report)
- Quick Transmigration: Fate Trading System (a former assassin is reincarnated in different worlds and has to seduce various women along the way -- in the current arc she becomes the companion/concubine to a female emperor)
- My Disciple Wants to Tease Me Every Day (a woman reincarnates and tries to save the world by reforming the female villain, who ends up falling in love with her)
- Considering the Possibility of Falling in Love With the Villain ( similar to My Disciple, but instead of a fantasy world its a post-apocalyptic world)

Can't believe I also forgot:
- I Favor the Villainess: A woman is reincarnated as the MC of an otome game, however instead of any of the male targets, her target is the female villain.
- The Villainess Blooms: Another villainess noble girl otome novel, however when the villainess is suppose to be punished by her prince fiance for bullying the game's FL, she's saved by the prince's younger sister. The MC of this one is the "villainess" who isn't a reincarnator or transmigrator, but it's hinted that the princess who saves her is.

sorry, it's not really reincarnation but more akin to isekai:
http://www.mangago.zone/read-manga/do_you_think_someone_like_you_could_defeat_the_demon_lord/
this one has reincarnation but it's not explicit shoujo ai (the female mc just has a harem and a reverse harem):
http://www.mangago.zone/read-manga/otome_game_no_hametsu_flag_shika_nai_akuyaku_reijou_ni_tensei_shite_shimatta/
I know this is an odd request but does anyone know any stories that have a shoujo ai (or yuri i dont judge) relationship tag that involves reincarnation?