Do you have a unique last name? If for instance your last name is something generic like 'Smith' or 'Brown,' other sororities may not realize you are so-and-so's sister in ABC. You also included that you are at a large school so that may go unnoticed.
This past year we had a few girls come through recruitment who were big time legacies (i.e. third generation from the school's chapter) and a few girls who had siblings in other houses. Several of these girls told us that they wanted to be given a fair chance and that they were not committed to joining the house(s) they were legacies at. I'd say it was 50/50 with these girls in terms of joining their legacy house.
I don't think it would hurt to be open about your concerns. Express how excited you are about seeing what each house is like and really wanting to find your perfect fit.
