What is there to be confused about? You W is her own person, your sister is her own person. Nothing, zero, zip, nadda, not a 2 pound terd from a hampsters tail can you do about what they do or don't do.
The only time I ever intervined with contact between (x)W and anyone in family was when everything got going with the A and (x)W was cooking up stories to validate herself to my 82 year old grandmother and it was breaking her heart. So I calmly requested that (x)W either control what she says or not contact my grandmother at all for fear of her health.
Me 35/XW 33 S13 & S12 M: 10/17/98 OM & S: 07/08 D final 06/09/09 12/03/09 - 06/13/10 "Piercing" 06/13/10: Engaged to Re-marry 10/17/10 06/25/10: Expecting baby #3 2/14/11