My 11 year old daughter humiliated her cousin in front of family at dinner I've told her she will be punished but can't decide what to do for the best outcome all round.

Hi I'm dad to two girls aged 11 and 6. My eldest started high school 1 month ago and has become very argumentative and has started to act like like a right little know it all.My niece 14 (her cousin) is visiting with her mother as it's the nieces birthday. Niece and her mother had recently had a row which resulted in niece receiving a smack which was reported to school and followed up with a social services visit yesterday.My daughter has heard about the row and follow up visit after overhearing conversations between myself and her mum. My daughter asked me what had happened and I explained the situation to my daughter. I made it 100% clear that this was a sensitive matter and was not to be discussed as my niece would be embarrassed if she thought we all knew what had happened.After dinner tonight my daughter was playing chess at the table while the family was gathered chatting and my daughter (in a major departure from her normal character) swipes all the pieces from the board, accuses my niece of cheating then blurts out that at least she didn't call social services on her mother. My niece ran upstairs humiliated and my daughter casually sits back and says "well, it's true!)I told my daughter to go apologise and she started crying so right now I'm asking what do you think is the best way to handle this as it appears I have some time before either will talk to me. via /r/Parenting http://ift.tt/2diQh6j