I'd join in on the suspicions if Brianna or Bruce thought Kristen was involved. But so far this is what I've found regarding their thoughts on Kristen:
His parents, who have been cooperating with police, back up her story that they were planning on moving and Smith was always planning to leave this morning.
Read more: http://wgntv.com/2014/02/07/search-on-for-wisconsin-newborn-taken-from-home/#ixzz2sefydgCK
She's 18, she's young. Apparently neither mom or dad were able to provide a home for Kayden without support from others. At 18 I was gullible and trusted people was too easily. We see people making that mistake all the time. I believe the stats of missing newborns is almost always someone wanting a child for theirself. I forget the stat's, but iirc, they are almost 100%.
My fear is that mom was too trusting and had a false sense of security because daddy is in the picture. In that case, an older person could sense vulnerability in both mom and dad here.
How and were did they meet KS? What proof exists she is related to BM's father?