What I overheard:

Evie: Andrew do you like that [birthday card I made you]?

Andrew: Yeah.

Evie: It's yours.  You can keep it forever.  Then when I'm grown up you won't forget me.

Andrew: Evie, I won't forget you, you're my sister.

The thing about this conversation you can't see in reading it is how sincere they were.  Evie was so excited to have given him something he really liked (it was a pop-up card I helped her make), and Andrew wasn't sarcastic or patronizing in his answer, but sincerely letting her know he couldn't forget her.  

I can never get over how great it makes me feel to know my kids truly love each other.  They are constantly doing kind things for each other and having conversations like this.