My brother fell foul of her, and lived to regret it.

She's on her last legs I don't doubt, or she wouldn't be in Zanzibar." "Then why the obvious nervous sweat you're in ?" demanded Fred.
"And that doesn't account for the abuse she handed out to us," said Yerkes.
"Why not tip off the authorities that she's a notorious spy ?" I asked.
"I suspect they know all about her," he answered.
"But why your alarm ?" insisted Fred.
"I'm scarcely alarmed, old thing.

But it's pretty obvious, isn't it, that she wants us to believe she knows what we're after.

She's vindictive.

She imagines she owes me a grudge on my brother's account.
It might soothe her to think she had made me nervous.