How sad!
I'm sorry that your brother in-law was so rude to you. Really, he may be a banker but he certainly wasn't polite or kind! Yes, the bank may be charging high finance fees, but they would not have let you leave the lot if you hadn't qualified for the car loan. Especially nowadays ~ banks are much more cautious in their lending decisions. Due to the financial crunch (mortgages being unpaid, etc.). You can therefore rest assured that the lender feels comfortable in your ability to pay off the loan.
I can understand your fear in the financial commitment. Fear that you might not be able to handle paying X amount of dollars per month for X years. That is normal. You are okay. If money ever does become too tight for you to make car payments, that's okay. You can always sell the car to another. You'll be just fine. Try not to let your BIL's rude remarks worry you. You are okay!
Shez