Someone posted KS is fairly recently married. Do we really know she had any kids?
My guess is going to be Ks couldn't get pregnant but hung around on baby forums. Started chatting with BM. Showed some sympathy after learning about BM's struggles/conflicts with her mother and offered them a place to stay... and some how they came to find they were sisters, with KS aligning herself with BM's dad's side as opposed mom's.
I think she was let in under the false pretense she was going to be moving mom, dad and baby across country with her, when all she really wanted was the baby and that is why she left when she did at such an unusual hour.