Janet, if you are on your sister's trusted list, that generally means that you are on the trusted list for her profile. You would need to be on the trusted list for ALL the profiles she has added for your family to be able to work on private profiles for them. When you mention the great-great type levels, those are probably open profiles, so any member can work on them without being on the trusted list.
This wasn't a recent change - trusted list access has always been specific for each profile. There is a way to do a bulk addition to the trusted list, for occasions like this one - you sister can do that for you.