I am a Fannie listing agent, and Fannie's policy is that in order for a purchaser to be considered within the First Look period, one of the parties on the contract must reside in the property after purchase.
In your situation -- your sister would have to be on the contract with you in order for this to happen.
If you're paying cash, this is likely not a big issue.
But, if you're getting a loan, your lender may have some restrictions on credit requirements for her being on the contract with you, etc.
If she's unable to be on the contract for any number of reasons, unfortunately, you'll have to wait through the First Look period and will be able to submit on the 15th day. I believe that HUD and Freddie take the same approach.
Let me know if you need anything else!