If he drove to his sister's house in the van, then why would she say that she assumed that he flew back? So maybe he didn't have the van when he stopped by her house, maybe he had the Mustang? She also alludes to the fact that she assumed he flew back because he ... and then she stops herself from saying more about this. Was she going to say that he flew back before?
On the other hand, if he drove to his sister's house with the van, then perhaps she didn't know that he just got to Florida, and assumed that he had flown home from the trip, and now had the van.