Okay, that is a very complicated case. The cousin usually isnt designed as a "love"interest. She is designed as a hate character, so you shouldnt be able to hire her via love option (even though the option is given). I am not sure how the base game handles her case, but in the mod, her schedule gets changed at different stages of her story progression. First as she moves into your home, second time when she moves out. And I think that is what fucked you up. There needs to be a way to either prevent you from hiring her via love option, or a safeguard check that prevents her schedule from being changed if you hire her before she moves out of your home. I will forward this problem to the Mod discord as well.