Is Riley the daughter of Marlene?
I'm interested in how they are going to handle combat in this game. Since.a big plot point of the story was Ellie becoming desensitized to killing; it wouldn't make much sense for her to commit many (really any, but I can see myself letting it slide for the sake of the enjoyment of the game) kills
The comics make it pretty clear that answer is no. Unless they pull some crazy twist thing in this DLC. Riley meets Marlene in the comics and neither of them know each other.
As far as combat goes.
Any killing of humans I imagine will be Riley's doing, but I don't expect much of it. I assume most of it will be luring people into traps or escaping. We will probably see Ellie with a bow, she did tell Joel at Bill's she was a good shot with one.