I don´t know what it means but Mercedes often opens her mouth and skims over Sophie´s back, not biting but just letting the fur slide through. Is it a scent marking thing?

I find tegan does that to blocks of wood (yes wood) when i bring them in from outside. She seems to want to learn more about the "smells" and "tastes" Does sophie go outside? Also has Sophie been spayed? Sometimes at the time they come on heat...other female cats will react to each other.

on the other hand Sophie could just be Mercedes "Favourite" lol.

From what I have read, usually a cat will do that with a scent that is not familiar with them. Perhaps Sophia has come in contact with a "foreign" scent and Mercedes is picking up on that.

She could be simply doing a special kind of sniffing. From Wikipedia:

"A domestic cat´s sense of smell is about fourteen times as strong as a human´s. Cats have twice as many smell-sensitive cells in their noses as people do, which means they can smell things humans are not even aware of. Cats also have a scent organ in the roof of their mouths called the vomeronasal, or Jacobson´s organ. When a cat wrinkles its muzzle, lowers its chin, and lets its tongue hang a bit, it is opening the passage to the vomeronasal. This is called gaping, "sneering", or "flehming". Gaping is the equivalent of the Flehmen response in other animals, such as dogs, horses and big cats."

Julius does this a lot if there is a new smell he is particularly interested in on something or one of the other cats he will smiff and then open his mouth as if analysing it.

My cat sometimes do this if they haven´t seen each other in a while. Like if I let one of them outside and they come back in the other cat will sniff and look at her like she doesn´t know who she is!

When Molly is scent testing he does the open mouth smell. Mostly he does it outside. If I see him do it indoors I tend to watch him, just incase he didn´t approve and decides he needs to spray.