Initially I thought SC was the children's primary babysitter but now am rethinking that. Since the SIL said they check the night before with whomever is sitting, it sounds like they might have several sitters and it could be SC might only sit 1 or 2 days a week. Another bit of information we don't know much about.
I feel like she was described as the "primary caregiver" to the grandkids in either an early article about the disappearance or perhaps in the that first podcast, The Missing. I will have to look for a link.
I think that with two parents, two busy full time jobs, there were probably other times - even if Sandra was the primary - when they had to get, maybe the SIL's parents, to look after the kids, or possibly another family member. So even if she was the primary, there might still be occasions when they needed to coordinate care and it was their practice to reach out to the person to confirm the night before.
One thing I found a little odd about his phrasing when he talked about calling to confirm the night before, though. He said, "She was always in tip top shape." Did anyone else hear this? I thought that was a strange way of saying "she was always available to watch them" IF that's what he meant by tip top shape. When I say "tip top shape" I'm thinking of someone who is alert, fit - it has a physical connotation to me, not just "available." So it seemed odd to me that he used that phrase, or that he felt the need to point that out.