Actions speak louder than words.Your relationship with your sister and her daughter seems to be very toxic.Their actions are too disrespectful. So many red flags here.You seem to be holding onto a hope that they might change one day.IMO they will continue to keep you in their life as long as you let them cross your boundaries and let them walk all over you or you keep being useful to them in some way.Non reciprocal, one way relationships based on selfish motives,do not have heart to heart deep connections.I think your heart is telling you the truth about your sister.These relations inevitably fall through.It is just a matter of time. See what happens once you put a simple boundary(saying no to dogsitting).I did the same with my sibling.They are so used to crossing my boundaries all the time that my first attempt to put a boundary did not go well.They became passive aggressive. It is not easy to recede from these toxic relationships. I realised it is necessary to do so, to keep my sanity.I feel like a fool now,the red flags were always there,the disrespect was always there,I always took the high road hoping for the change which never happened. It looks like to me you have taken enough toxicity from them.Hugs.