Those family relationships would not qualify you as a legacy for most NPC sororities, if not all. However, these family members should write you a reference and send it to the chapter of the sorority at your university. These would be very strong recommendations because they come from people who know you well. As someone else mentioned, because they are family members, it will show you understand a little about Greek Life, too, because you have had the influence of sorority family members. From there, the sororities will presume that because of this influence, you likely possess some of the characteristics that would be favorable to a sorority sister.

References and legacy status don't guarantee a bid of membership, but they are soft factors that help you stand out, just like your extracurriculars and community service helped paint a picture of who you are to college admissions boards. You're more than a pretty face with a good GPA! :)