Agreed, dogs need consistancy! And if your the primary carer of the dog then ofc it will respond most to you...its common sense! am i right in thinking your sister is 24? Sounds more like she acts like a 10yo... if they want to bond with the dog then the effort needs to be there from them..training/walking/playing...the whole lot.
Most dogs wont instantly bond with everyone they have contact with, it take time and work to make that bond!