Meaning of "my fool of a sister can't get at you"

Question

Another phrase/sentence I don't understand from Narnia Book 1 Magician's Nephew:

Digory was quite speechless, for Uncle Andrew looked a thousand times more alarming than he had ever looked before. Polly was not so frightened yet; but she soon was. For the very first thing Uncle Andrew did was to walk across to the door of the room, shut it, and turn the key in the lock. Then he turned round, fixed the children with his bright eyes, and smiled, showing all his teeth.

"There!" he said. "Now my fool of a sister can't get at you!"

Don't understand completely. I understand every word, but I don't know what he is talking about.

Answer 1

He's saying that his sister is a fool. "My fool of a sister" means exactly the same thing as

"My sister, that fool."

It's a very common construction in English.

Some more examples are:

That idiot of a president. That pile of junk of a car. That jerk of a boss.

Each is simply identifying the noun before the word "of" with the one after.

edit: "Can't get at" means can't access. So he's keeping the children away from his sister, who is a fool.

Answer 2

"Can't get at you," means "can't harm you." Basically, it means "can't affect you." But it's doubtful that Uncle Andrew would refer to his "fool of a sister," (i.e., his sister, the fool), if he thought that she'd affect people in a positive way.

Answer 3

I haven't read the book, but I am assuming "Uncle Andrew" is living with his sister, and the children have come to stay with him. I am also assuming that the children have discovered something Uncle Andrew has done, and so he is locking them up. Hence, he is saying that is "fool of a sister", or "sister who is really dumb", can not "get at you", or "save the children".
She might not even know where they were.