This is a tricky one. In the context of financial domination as it is done here it is a sexual fetish. So the idea of a mothet or sister doing so does have somewhat incestuous overtones.
i am trying to think of a more articulate way of expression than eeeeeewwwwwwwww.
On the other hand it does demonstrate a definite commitment to the ideal that the Female is dominant. Now i am perfectly comfortable with the notion that a slave, a true slave has literally no choice or rights ONCE they have made a commitment to that philosophy and lifestyle. But to be denied the right to choose this lifestyle to begin with i consider dubious to say the least.
And being dominated by your sister, what happens if you don't submit? Genital torture hardly seems appropriate.
And let's not forget that they are family. Do they enjoy the connection, support and hopefully love that family members should.
There's a lot of grey areas here need clarification.
At the very best I'd say it should be good for a few episodes of The Jerry Springer show.