Here's how most sororities phrase their policy towards legacies: they don't have to accept the girls, but they usually do have to invite them back to the second round of rush, and if they invite you back on Pref night (the last night of rush), they have to put you on their first bid list (a complicated system, but basically what it means is that if you're on their first bid list and you rank them first, you will get a bid from them). But they can drop you in the middle rounds of rush, so they're definitely not usually pressured to invite you back if they really don't want you there.
Of course there are sometimes special circumstances. If you're going through rush at a cut-throat school, they might drop you after the first round. And if you're the great-great granddaughter of a founder or something, they might be feeling a little pressure from sorority headquarters to give you a bid no matter what.
But under normal circumstances, both you and the sorority will be given a good opportunity to see if you're right for each other.
Good luck with rush and keep us posted on how it goes!