Isn't she the one exploiting them more?
She's benefiting off the chance that there is nothing between Saito and Akane and using his obliviousness to have some time being close to him.
I know Akane's reaction was over the top but having the school know that they are living together is annoying and I would understand why people who are (at least initially) against a forced marriage would keep it a secret. Normally I'd hate Himari in fact for messing things up so badly tbh so this is me letting her off easily.