Even among the closest of sisters, there must be rules. Which of these two lovelies do you suppose is the Alpha Dog?
She's the one who rules the roost. She's the one who takes the food first. It is she who must be petted first, and if she feels that's not happening, she positions herself firmly between the petter and her sister.
She would be Lexi, she of the white nose.
Samantha, of the black nose, is the submissive one.
I always thought that Lexi was the boss because she was  the one who did the humping while Samantha endured this public humiliation. But then we were not sure due to a recent incident.
Monica believed that the poor girls needed some new toys. Pet Smart has an aisle set aside for Monica. Her name is on it. It is there that she gets the "indestructable" ropes and rubber balls and stuffed animals that Lexi and Samantha proceed to destroy within minutes of the toys' arrival in the house..
I put my foot down. No more unwise spending at Pet Smart. I'll just get some good, strong beef bones from the butcher and give each of the girls their own.
We did that, taking the girls outside and handing over the bones.
Within minutes, Samantha had gone berserk, growling at Lexi with her bared teeth at her sister's throat.
We took away the bones and went to that fount of dog psychology -- the Internet.
What we learned is that when the non-Alpha Dog feels threatened, they will strike out. Also, when the "parents" are trying to dispense things in a democratic fashion -- be they bones or affection or toys -- that's not good dog psychology.
Dogs don't like democracy. They want to know who is boss. It keeps everything simple.
So now we've been letting Lexi out of her crate first, giving her her food first, petting her first, and domestic harmony seems to be prevailing.
We just won't be handing out any more bones, no matter who is the Alpha Dog.