(((((((Twinmommy)))))))) Just a question, hon. Why does your brotherinlaw care, if his brother (you hubby, right) decides that it's a good idea? I could see him being exasperated, but he is not in the position to be "mad" and to try to bully you into doing things his way. That was, in my mind, very out of line. If he had doubts, there are other ways to express them.
My suggestion, refuse to talk to him on financial matters. If he and your hubby want to talk, that's great, but it seems as if he uses these times to get something other than what's even good for you, let alone best. I have in-laws that used to play the same game (it's always the woman's fault, see) and I finally simply refused to speak to them of such matters. I'm very glad I did, and I am so sorry that your brother in law made you feel so bad.
Be happy about the car, what will be, will be, and you only make yourself feel worse immagining the worst. Doing so will not help you prepare for it. Who knows, it may not be a relevant matter tommorrow, even. Nothing is sure, so enjoy having made a choice and getting a new car for right now. You're not stupid, and even your hubby agreed with you. Hugs and take heart