ESH I'd say you were not TA but you admit what you did was a bit much. Your family is definitely in the wrong for habitually taking from your share of any treats you had set aside for later, but you could have handled it better. I don't really blame you too much if it had been building up, but if you want them to listen and change their behavior you would be better off apologizing for the outburst and then adding that it is still wrong for family members to keep taking your share of the treats, and that you think you have a right to demand that they stop.

Ask them what they think is fair and how you should handle it if it happens in the future. If they aren't complete AHs they will cede that you have a point.