It's not about the "methodology" of things being easier, it is just "convenience" for them. This is just a very simple "first world problem".
Your daughter didn't even need to go with you in the first place and it is a strain on yourself and your daughter because both of you will probably be bored and did not really need a trip to the dentist.
In the end, you could of just told your sister to shove her idea and let her deal with whatever "problem" she had.
It depends if you want someone to guilt you as your "duty" of being an aunt is to look after your nephew at all times and keep him safe.
If your nephew really did need to see the dentist because it was a serious problem then I am glad you did so. If not, then tell your sister to stop being so freaking paranoid and don't let her guilt trip you into doing things she wants because it's easier on her.
The DAD has even more "duty" as a father to make sure he is the first point of call to help his son out in an emergency. It all depends on the circumstance of the family, but if the mother is unemployed...THEN SHE HAS ALL THE TIME IN THE WORLD TO CARE FOR HER CHILDREN !