I'd love to ask Cousin T a few questions, and I think the first would be - how did you leave it with Sandra that afternoon? Did she say she'd think about it and let you know only if she decided to come over? Or did she just say she'd call later after she'd dropped the car off? And if she said she'd call later and didn't, why didn't you call her when you didn't hear from her? Did you leave the ball in her court? Was Sandra in the habit of just blowing you off without confirmation of plans?
Along those lines, when SIL knew he was running late, why didn't he give Sandra a call to let her know what was going on? The lack of phone calls even before the disappearance is so strange. Unless of course, Sandra just never answered the phone, so her family didn't bother to call. That's the only thing that might make some sense. Not that never answering your phone makes sense...