I think you need to tread carefully with your stepdaughter here... she made some extremely poor decisions, but how you handle it can make it better or worse!
The wrong people are being punished. Because of her actions, the other two got a worse punishment -- that's crazy!
I think you need to correct the behavior, not just blindly punish her. (You need to discipline her and help her see why it's bad, and how she can make amends, and make her NOT want to do it again).
I would:
Have Stepdaughter write a letter to the principal saying she believes the wrong people are being punished, and that this was all a result of her poor choice to bully someone. She should offer to serve her detention PLUS 10 days of internal suspension in place of her stepsister (5 days) and the girl she was bullying (5 days).
I would also make her go to Clara's house, in the evening with Clara's parents present, and have her give a heartfelt apology to Clara. She should bring a gift that she has to pay for. She should have a written apology to leave behind. She should also apologize to Clara's parents.