Anna doesn't move for a while. Doesn't all this time, it still forces her to her metaphorical knees.
Elsa doesn't need to know. And Anna trusts that she won't mind, either, if she doesn't. But... Anna can't. Or, she won't. She wants to share that with Elsa. She wants to share... everything with Elsa...
"Olaf. My little brother," she eventually murmurs. "You've seen him, already. That old photograph."
Elsa's eyes widen in understanding. She wants to ask, but fears the worst. She was right.
"My little brother and I... killed him..."