Since your brother had your permission, you will be held liable. If the claim letter is from an attorney you can give them a call and request the proof of the damages and then determine if the estimates are out of line. You probably can agree to a payment plan by talking to that attorney who is doing the collections for the insurance company.
You need to take your paperwork and consult with an attorney in your state before you sign anything. It is difficult to construct an answer based on the question you have presented. Do not sign anything until you talk with a lawyer in your area, if you do, you do so at your peril.
I suggest obtaining representation. I agree with the other attorneys and feel the other person can come forward and testify that they were the one that was driving at the time of the incident. But since you did not have an attorney in the beginning, you will have alot of work undoing the damage that has already been done.
If the van was ticketed for following too closely then I would say yes filing a claim against the van's insurance should provide coverage for your bills. Your deductible could be worked out with the other insured's insurance, it just depends.
That is a difficult one to answer with the limited amount of information. If the total medical was under $300 I assume the treatment only consisted of diagnostics? If you could provide more information it would be helpful. There is no clear cut formula to determine the amount you should be compensated for. Alot of the compensation has to do with the type of bodily injury, which you have not specified.
If your name is still on the title as the owner of the vehicle, then I would that technically you could be held liable. If nothing else, if a lawsuit is filed you could be listed as a party in the lawsuit and then have to prove why you would not be liable for the injuries of the Plaintiff.