D.K.
Hi. A two year old that is old enough to fight with his sister is old enough to understand that his actions have consequences. Make sure you get down to their level when you correct them and use a firm, "I'm not playing" tone to explain in terms they do get that they can't do whatever it is they did.
When my 12 and 10 year olds were much littler I would ground them from each other when they bickered too much. Kids always want the things they can't have, and it was surprising how soon they were begging to be around each other and then they treated each other better.