Amilyn allows herself to be led, and before she can answer, she has to sit down.

"I have. He—" She's still processing that conversation, still unsure what she has taken away from it, and not at all certain of what she wants to tell Leia about it. "Leia...I thought—thought he'd been dead, all those years."

It's not why didn't you tell me, because Amilyn doesn't want to be accusatory. But the question is implicit.