Chances are, the members of AKA at your school already know you are a legacy. Sometimes the legacy's family members will write a letter to the national chapter or school chapter letting them know you are enrolled in school there and are thinking about rush. As a former rush chair, I would recommend you do mention (very casually) that your mom, sister, etc. are all AKA's and that is why you are so interested in rushing that particular sorority.That way, in case the sorority does not know you were a legacy, you will have alerted them just in case they need to do any kind of paperwork if they give you a bid or if they reject you. Yes, there is paperwork!
Good Luck.