My initial reaction is, its not you, its your sister. YOU are the one with a commitment to her, she knows she can trust you, but she doesnt have that deal with your sister. Thats why you are exactly correct when you say your sister needs a t. Your sister is accountable to no one. You and your t ARE accountable to each other, but your sister is not part of the deal. So its like shes saying, you cant change boundaries on her either (to include your sister). KWIM? (My first thought about this).