Mubarak Family Massacre
Perhaps it's not a typical massacre, but a family in their home was knowingly targeted in Idlib province, June 18, in a rocket/missile attack. Apparently there is contention over what happened, but that comes to us seemingly pre-solved by the Syrian Observatory for Human Rights (entry below); it was a rebel crime to eliminate a loyalist family of some prominence.
- Idlib province: 20 people have been killed by a Grad missiles attack on the house of Ahmad al-Mubarak, member of Parliament and head of the Bani Izz clan, in the town of Abu Dala, 40 Km south-east of Ma'arat al-Nu'man. Opposition activists are claiming that he was killed by regime forces, and that a fuel tanker was by in his house and caused the explosion. But it is worth noting that Ahmad al-Mubarak is a well-known regime supporter, and that one of his aids was executed by rebels only a few days ago (9/6/2013). 
- Idlib provinceSyrian Observatory for Human Rights, Facebook post, June 18, 2013