One cannot 'Control' another person. One can 'Guide', 'Help' another person through a tough time.
As for the one being 'Bullied', they have to figure out a civilized way of dealing/handling the situation.
The 'Bully' is usually trying to make friends and fears being rejected and has already assumed that they are before they even ask.

I think i would try to talk to my 15yo in language that makes sense to her about the importance of the role of a big sister - to show kindness, helpfulness, to boost a little sister up so she feels smart and safe and respected. She can choose to help her sister be awesome rather than feel lousy. I would ask her what kind of things she would do or say to help her little sister feel smart and safe and respected, and I’d give her examples if she doesn’t know.

For my 13yo I would talk to her about drawing boundaries, saying No/Stop, and about how she is an entirely separate and individual kid who can choose who she wants to be and how to act. In a few short years she will get to know the world without her sister around all the time, and the world will get to know her. She can be her wonderful self now, and tell her big sister to zip it if she’s being mean.

Maybe there are age-appropriate books on the issue.