I think it’s kind of refreshing to have a harem member be an antagonist to the other ones. It’s even more spicy when it’s the one that everyone considered to be the oneesan who looks after them.
She used to feel a bit like a mob character who went with the flow before. Now that she embraced her evil ways she really came out of the cocoon. A potential cause for it might be resentment from always putting the well-being of her sisters before hers, until Fuutarou came.
Her misplaced feelings and newfound love could have been the catalyst for her change. I wonder if Yotsuba will also experience a similar awakening and how she’ll deal with it...