how can i stay away from a house if i’m a legacy??

To my knowledge you don't have to report being a legacy. You shouldn't worry about being stuck with a group you don't want, though. As someone who has done rush as a member and seen it on the other side, sororities can tell when someone is uninterested and will most likely drop you for that reason. If you make it to pref night at a house you don't want, there's nothing wrong with politely telling them you feel like you connect more with your other house and going ahead and suicide bidding.

you don't have to report being a legacy! And don't be afraid to show them you're more interested in other houses but please please please don't be rude about it. I'm in Kappa and we get a lot of PNMs during fall rush who are unnecessarily rude to us because they don't want to be in a low-tier house. It's totally okay to subtly hint that you are leaning somewhere else, sisters will recognize if you aren't thrilled to be there, but please remember that these girls are required to be nice to you and work really hard for months to present their chapter. Also, after you are a sister it is likely that you will meet and become friends with girls from a wide variety of houses so make sure that you are nice to everyone for that reason- when you join other clubs/meet people in your classes you don't want to have burned bridges just because you didn't like someone's house during recruitment.