Why does my puppy bark at my brother but know one else in my family?

if me or someone in my family goes out and we come back and she sees us coming up the front path she will watch us and then wait by the front door, but if my brother or someone she doesnt know walks up the front path she goes ballistic. she will growl and bark. and when my brother got in at 3am this morning she started barking her head off, but when my dad goes in from work late she doesnt make a sound