Bingowings said:
Also for the last time Vader also assumes that Luke AND Leia have some innate strength to destroy the Emperor.
If Luke doesn't turn he will turn Leia (Vader doesn't make empty threats).
Sure he is hoping this will snap Luke and turn him to his way of thinking but if it doesn't Leia is clearly Plan B.
Even Luke says so.
He says to her that she is the last hope for the alliance because she has the magic wish power too.
Not Admiral Ackbar or Lando but his sister.
She has the ability (according to him) without Ben or Yoda to train her.
It doesn't have anything to do with inheritance.
Vader actually experienced/felt Luke's strength in the force. He didn't make any assumptions.
When he said to Luke that perhaps Leia will turn if he won't... it doesn't explicitly say that he will use her powers nor that she is even strong in the force. It was just a threat.
When Luke says to Liea that she is the only hope for Alliance, he doesn't make an assumption that she inherited force powers. He too experienced/felt her having them.