I have this rrat that they're both aware that Peanuts-kun would have not been able to become successful if he didn't have a sister.
Ponpoko uses this to blackmail and abuse Peanuts-kun. As a sex slave to relieve her lust too, sometimes.
That takopii episode was actually a cry for help.
https://www.youtube.com/watch?v=QyKHBmr_Zps