Okay it is weird that he's dating his cousin but remember if he doesn't date her then she'll die but still it's weird but it's 100% legal just would be weird and unpreffered.
Copy and Paste in URL.
Its all Yuri.
https://mangabuddy.com/manga-list/85731
http://vyvymanga.net/collection/618453
https://mangadex.org/list/b92507b7-3649-4cab-8a5e-3641c184fc6a/yuri