Say What?

I went to pick up the kids this afternoon from my wonderful Sister-in-Law and this is the exchange that followed between the kids and I:

Me: Lyssa, come get in the car sweetie.
Alyssa: *wanders around to the other side of the car where said SIL is putting her sister in the car*
Me: Lyssa, come here and get in the car. Brother is crying. We need to go.
Alyssa: *follows SIL to her car*
Me: LYSSA. COME HERE.
Alyssa: *starts crying*
Me: *walks over and picks up child to put her in the car*
Alyssa: I want her. *motions to SIL*
Me: *dies a little inside, but puts her in the car anyway*
Alyssa: *continues crying*
McKenna: My heart is breaking a little too, mom. I'll miss Jessica.

Where do these children come from and where do they pick up these phrases?