Responses
She doesn't want her reputation to be from being Daisy's sister. She wants it to be because she is Rose.
We've already seen 2 examples of how ruining Daisy's life might be beneficial to her.
She is not stupid, she's just a bitch
Not only is she too dumb to realize that Daisy marrying up means that now her prospects rise, she's also too GD petty and short sited to see that any stain on her sisters' reputation will extend to her too. That's how family reputations work. If she thought a decent marriage was hard as a poor girl, she really wouldn't have stood a chance as a poor girl with a ruined sister