Surely the brother will have to declare it and pay tax on it.
I can understand why someone would want to do this - benefits like UC will stop very swiftly and next time she claims, her spending would be scrutinised very closely and that scares people.
In her shoes, I would work out what I want to spend it on, say house deposit, repairs, new vehicle, new white goods etc.
All of these are reasonable things to spend large sums of money on, so would not be penalised.
Then I think she can put some of it into child savings, Junior ISA's etc (She'd need to look up which ones were ok as far as benefits are concerned, she can't just give it all to her kids).
Then yes, whatevers left over she'd have to suck it up and live on that until it runs out - as long as she keeps receipts or uses debit card to pay for things she will have a paper trail to back up what shes bought and as long as she isn't silly, and doesn't go round buying things to sell on or gifting money to people, she should be fine and can re-claim when it runs out.
If she gets caught, and she can... hiding these assets in someone elses bank account, then yes she will be in deep shit and will have to pay back benefits, may not be able to claim again for some time.