What is wrong with my son?

My 15 year old babysitts his 11 year old little brother for an hour after school while i am at work. The 11 y/o has been telling me his brother is slapping him, teasing him to no end, spitting on him, wripping his homework up, and making him cry for the past three weeks! I just found out about this yesterday and he has hasnt done any of this before. I tried talking to him about why he does those things to him but he started yelling at his brother for telling me and my younger son started crying. I grounded him for a month. I am so mad at him for hurting his brother. I am also going to have us all go to councleing. Is there anything else i should be doing about this?