My theory this many hours later. If this car was really carjacked the child would have been found hours ago. From memory when a car is found the child is found not too long after.
My theory,and I hate to say it, but I wonder if dad and step mom didn't like how Bianca played with her 2 yr old half sibling. Many of us remember how two two year olds play next to each other. They don't really play together. They get fussy over who gets what toy with one seemingly only interested in what the other one has.
I hate to say it but I think we're going to learn this was originally supposed to be a much shorter visit. Something happened and dad and step mom had to keep delaying returning Bianca to her mom. Which would explain why she needed more clothes. When mom finally said enough is enough, I want her home TODAY, or tomorrow if she made this demand last night, they had to come up with something quickly, hence the 'car jacking'.
Someone posted upthread how common it is for carjackers to ram the rear of a vehicle. Come on, we've known that for well over a decade. I might think an elderly person might fall to that kind of 'scam', but not a young man in his 30's. Nope, this was a quick ploy they concocted when they couldn't return Bianca to her mommy.
I hope I'm wrong........
I agree with your theory. Bianca was big/tall for her age and spoke well. She may have taken something from the smaller (?) child and the father went beserk.
However, Bianca's mother and uncle said they talked to her the night before she was reported missing. They didn't say what time, so it had to have happened after that. I imagine the woman is being questioned, too. jmo