Most likely the SIL was offering what comforted her.
I guess the three options are that Ren's daughter asked the question because it just occurred to her without any input, someone had suggested the idea to her directly, or she had overheard it. I think it's most likely that someone spoke to her beforehand in an attempt to comfort her, but it doesn't matter in the end. I continue to think that Ren did the right thing, and a child shouldn't hear that a parent wants to go on and leave her behind. Even a religious person focused on self-comfort during such a difficult time should realize that a child is part of the picture, and also that it's almost certain to be untrue that a dying parent
wants to go to eternal rest while leaving a small child behind. Such statements where children are involved are at best false and cliched, and may be inconsiderate and hurtful.