I noticed in one of the recent articles about Sandra's vigil that her daughter was quoted as saying "we dropped her off." That really stuck out to me as everything up to this point said only "family member," which really could be anybody in a very extended network of people....sister, cousin, niece, etc. We know from other articles that Sandra's daughter was at work in Boston during the time that Sandra needed to drop her car off, so whoever the unnamed family member is, it's someone close enough that to Sandra's daughter, they come under the umbrella of "we." I also believe, based on the timelines provided by MSM articles, that the three children were in the truck when it stopped at Cumberland Farms and were transported by that driver back to their home after dropping off Sandra. So it has to be someone trusted to drive the children, and who also has the space in their vehicle and ability to install 3 carseats (not easy to find just any car where this is possible to do safely). All of this leads me to conclude that the unnamed family member is actually Sandra's son in law, who would normally be driving the children. Why they would not name him, or whoever it turns out to be, I have no idea, unless the police department advised against it for the very reason that is now taking place...the sleuthing and suspicion that falls on the last person to see her.
I think that the identity of the driver is likely a distraction to actually solving the mystery of what happened to Sandra, as police have had a lot of time to verify the driver's account and timeline of the drop off. There may not be security camera footage that recorded her entering her home but I'm sure there must be plenty of business and security footage of the route the driver would have taken upon leaving Sandra's neighborhood and returning to the children's home in neighboring Pembroke.