I always tend to think Luke was the patsy, left in plain sight so that he could be found, Vader finds his son, has no clue that that Padme had twins, and either kills or turns him to the dark side, once that happens Liea gets sent to Yoda to train near the dark side cave in dagoba where the aura masks her own force presence, until she is strong enough in the force to tackle Vader herself.
Of course she messed all that up by getting herself captured by Vader, so they had to go with plan B, activate Luke instead, while scratching their heads as to why a person living on the Skywalker farm with the name Skywalker was never captured by the one person who would get the connection.