Imagine this: they have to make sure that Sanji won’t bail out of the wedding and they know that the bracelets and Zeff won’t be enough, they need another reinforcement. Knowing they already have Cosette right under their thumb, they force her to be Sanji’s company, monitoring him so he won’t do anything.
Sanji is aware that they’re not handing him a handmaiden of sorts just for laughs, and is just polite to her, but slowly, they start to get close.
As in “I’m starting to fantasize you and compromising everything I’ve worked for for a future with you” close.
Bonus: they might start a physical relationship ( ͡° ͜ʖ ͡°)