The relevant part of the Torts (Interference with Goods) Act 1977 state:
"If the bailee [your mother] . . . has failed to trace or communicate with the bailor [your brother] with a view to giving him such a notice [regarding collection of the goods], after having taken reasonable steps for the purpose, and is reasonably satisfied that the bailor owns the goods, he shall be entitled, as against the bailor, to sell the goods.
. . .
A bailee exercising his powers [to sell the goods] shall be liable to account to the bailor for the proceeds of sale, less any costs of sale, and the account shall be taken on the footing that the bailee should have adopted the best method of sale reasonably available in the circumstances"
So if any of the items in the shed might be 'saleable' it's important that some effort to sell them is made (rather than, say, simply donating them to a charity shop) as your brother would be entitled to the proceeds of any such sale (less any expenses occurred in making it).