several thoughts from a long time auto adjuster.
First, the liability is your brother’s. The motorcyclist owns the lane he is travelling in, he is free to speed up or slow down as he sees fit, and it is your brother’s duty to yield to his right of way. The ONLY way your brother could get out of it is the “but for” argument, if you could proove the accident would not have happened but for the excess speed of the other driver.
Second, the apportionment of liability. Your brother is liable for the accident as the driver, you are vicariously liable as the registered owner. In most states, vicarious liability is capped. More than likely, the amount you would be responsible for is capped to an amount below the limits of your liability policy - meaning you are bullit proof. Your brother is more than likely the one who should be loosing sleep, not you.
Thirdly, your insurance company has the duty to indemnify the other party up to the policy limits, or defend you (at thier cost). They do not defend you up to your policy limits, they defend you. The claims adjuster will either reach a settlment within the policy limit, or there will be a suit. The other party can ask for all they want, only a judge can order a judgement in excess of your policy limits. Some thoughts on this UNLIKELY event. You are blanketly covered by the vicarious liabilty rules of your state. A judge will look at your brothers assets and ability to pay, and he will look at the other partys injuries and impact on life and put it in perspective of any comparative or contributory liability issues. For a judge to issue an excess judgement, the injuries and impact have to be huge (read catastrophic), the liability needs to be extraordinary (more than the garden variety mistake anyone could make), and the person needs to have a capacity to pay. There are possibilities for an excess judgement here, judges don’t always but usually consider the factors i pointed out. In the end, you and your brother met the minimum liability insurance requirements of your state, and it is not that easy to get a judgement in excess of these limits.
Lastly, In the highly unlikely event the other party does not accept your policy limits, and there is a misalignment in the stars and the judge indicates that he would consider excess issues you can get your insurance carrier to pay for your own personal lawyer to defend you for the possibilities of an excess judgment, this is called cumis.
Good luck, and you are needlessly worrying.