Step brother hitting his step sister?

Hi all! We are a mixed family with 2 kids coming from my husband, and one from myself, and one together. Our 13 year old boy (his son) lashed out at my 8 year old daughter and hit her hard! Our 7th grade son is about 5'6 180 and our 2nd grade daughter is about 4' and 50 lbs. This really upset her and hurt her. We have no idea what a proper punishment is...but this is just NOT acceptable. Of course we grounded him at the moment...but how do we teach him this is not ok? Our 8 year old did say something she probably shouldn't have to him...but it was all in joking and everyone else knew that. For him to turn around and clock her one is just not ok. What would you do in this situation??? I have a tendency to have a temper...but have to walk a fine line with the stepson.