As per the others, I suspect you are confusing this issue with either corporation tax liability or inheritance tax liability (on your brother's estate).
Firstly, the company is liable to pay unpaid corporation tax on any PROFITS for any years not covered by annual accounts. The small business CT rate is 20% - a bit of a coincidence that it's the same figure you mention.
But this is nought to do with assets in the business, which are owned by the shareholder(s). If the sole shareholder was your brother, then the assets of the business once liqidated go back to the estate of your brother, and may increase the liability to inheritance tax (IHT) on his personal estate (subject to nil rate band allowance being used up, etc). But the rate of IHT on the amount less allowances is 40%, not 20%.
So please explain what you mean, and who the shareholders are of this 'family business'.