Try drawing her attention to the consequences, while giving minimal attention to her. A line like "Your sister is upset because you pushed her. I'm going to have to cuddle her for a long time until she feels better." (while cuddling the younger one, avoid eye contact or conversation with the older one, just talk to the younger one)
Build on this by giving the older child lots of opportunities to be a good sister, and heap the praise on when she does it. Use opportunities like "Can you help your sister to get her cup/blanket/shoes/crackpipe/whatever?"

See how that works.