The jacket may be the identifier. We all know that Gannon's favorite color is blue.
Also, would SM expect his little sister to shut the car door, or would she close it after her? If Gannon was drugged or sick and she helped him up but his legs were still hanging out, I could picture her telling him to get his legs in and shut the door, you know?
To me, while it could be the little sister being put in the car, after looking at the tiny portion of video we were given including the color of the jacket and the fact that she left the door open, I feel like it was Gannon getting in. And I think his dad would be able to tell better than any of us could.
The other question still remains though... why did she leave out this trip when she was talking to LE? Why did she say Gannon left on foot to go to a friend's house, yet we have no surveillance reported where he was walking out the door and down the road?
There are too many questions unanswered for me to personally let SM off the hook.
All of this is totally IMO!