I didn't read that in, but it may have been implied and I just missed it. It seem to me she was so under his spell and vise versa that they weren't going to take no for an answer, even if it meant drastic action.
I think she was in on it, maybe all of it including the murders. My concern is that, if I'm right, and they let her off on this because of her age, she'll do something awful somewhere down the line. It takes an especially cold person to stand by, let alone help plan, the murder of one's own parents--if that's what she did.
I guess besides her talking about "the risk" they took, I read into the language, "David and I are going to take care of it."