[Help] How can I get my dog to obey everyone in my family?

Sup, DoggitMy dog (almost 2 years old, Labrador Retriever) is pretty well behaved when I'm around or when I give it commands, but doesn't really listen when the other people in my house give her commands (even when using the same words as me).For example, sometimes my mother will tell her to drop something and she'll just stare. Then when I tell her to drop it she drops it.I was discussing this with my brother and sister-in-law and they said that she has to listen to everyone. My sister-in-law grew up around and her family raised a number of dogs and said that the dogs she lived with always listened to everyone in the family (a large family).So my questions are:Is it normal for a household dog (so not a guard dog or a police dog) to only obey 1 person? (research or professional opinions here would be great!)Normal or not, is it possible to teach my dog to obey everyone in my family?If possible, how can I teach my dog to obey everyone in my family?By "everyone in my family" I mean my mother, father, brother, and Sister-in-law, fyi.Thanks in advance for your help! via /r/dogs http://ift.tt/2rMKSf9