If I were in your place I would have felt the same. Are your mother and sister always like this? Do they like to rub salt in your wounds or are they supportive otherwise? If it is a one time lapse then you can cut them some slack and look at the bigger picture, as eskielover has suggested. If they are repeat offenders you can give them a stern talk. Either way, you are not overreacting at all. They should have kept you in the loop when inviting in your ex.