They'll find that bastard Mr. Blish if it's the last thing they do.

It's unclear why this is a frightening concept. They’re out in space, so they can’t do any trouble. Though the wicked sisters ( I presume they’re sisters; that's usually the case) may be able to fly around without suffering decompression, they’re too far from Earth to cause mischief through their necromancy.

The house is not connected to any utility lines, so they cannot disseminate poisons through the municipal infrastructure.

If they attack any spacecraft, one can assume it will have sufficient means to defend itself. It may not be able to kill the witches, but it could probably blast the house to pieces.

And then where would they be? Then what would they do?