Sibling dynamics

My wife and I have been separated for 18 months but are nearing the end of a one-year reconciliation program so I pray that healing is near for our family. We have four children 12, 10, 8 and 4.

My 12-year-old son and 10-year-old daughter have developed a relational dynamic where he nonchalantly makes uncharitable comments or critical observations about her, and she becomes livid and defensive while he ignores her distress and continues to dig in. This past weekend, I encouraged them to use TSG principles to settle their conflict, especially disagreeing appropriately. What advice do you have for me to both instruct them on, and for me to intervene with consequences? In the most recent case, I assessed an extra chore for my son because he was teasing and did not stop when asked.