Have it to where no one (even inquisitors) knows vaders real name.
She's already met Owen, I can't remember if he said his last name or not. If not add it in.
They know that Vader wants obi Wan, but they just figure it's bc he and Yoda are the only living members of the jedi council.
In the 3rd episode when Vader shows up, obi Wan sees this bad ass sith lord but doesn't know its anakin. During their fight, Vader mentions how rusty obi Wan has gotten since he quartered him and left him to die. Obi Wan then has the flashback to anakin crying in the lava, and says "Anakin....skywalker?" third sister is listening from the shadows and realizes the possible connection to Owen, but she recapture Leia, so puts it off for later.
After they escape again in episode 4, you can have 3rd sister go rogue and return to tatooine where the final showdown is over Luke. She doesn't tell anyone else where she is going, so when obi Wan eventually kills her, Luke's secret is still safe from the empire.
For this to work though, Luke would have to be unconscious, in Carbonite, or have his memory force-wiped, so he doesn't remember it in episode 4.