More bibliographical info: Lycanthia, or The Children of Wolves, NYC: Daw, 1981.
This one is a bit more circuitous. First, the explanatory passage from DS6:
Looking dubious, Sirius retrieved the glinting object held it up to the light. It was the silver pendant that Slytherin had tossed to Draco, that had acted as a Portkey, although there was no way Sirius could have known that. He glanced at the odd shape of it-the sideways X, almost, but not quite, a cross.
He walked back over to Lupin, who was still sitting on the floor, cradling his arm. Sirius knelt down next to him, holding out the silver X, but Lupin shook his head. "I can't touch it," he said.
"It's a Lycanthe," said Lupin. "Old magic. Protection against werewolves."
"I thought it was a crucifix," said Sirius, looking askance. "Crucifixes don't bother you, do they?"
Lupin looked aggrieved. "I'm a werewolf, not a vampire," he snapped. "It's a Lycanthe, like I said. Not a crucifix. Totally different. Very, very old magic."
"Strange shape," said Sirius, turning it over in his hands.
"Not really," said Lupin, and smiled a funny half-smile. "Say you're walking through the forest at night," he said. "Alone. No help in sight, and you don't have a wand. Then a wolf jumps out of the darkness, straight at your throat. What do you do?"
Without thinking, Sirius threw up his arms - one across his throat, the other crossed over it, protecting his face. Making a sideways X.
"Right," said Lupin. "Lycanthe. Old magic. Like I said."
Sirius blinked and lowered his arms.
As a slight nitpick, CC's memory seems to have slightly blurred here with another one of Tanith Lee's werewolf tales, "Wolflands" (first pub. 1980) in which symbols are scratched into the snow to summon and form a pact with the wolf-goddess, not to keep werewolves away. In Lycanthia, the term "Lysinthe" [sic] is variously applied to the wolf-goddess herself (p. 187; older forms of her names are "Lukanthis" and "Lycanthia"), as well an awkward gesture of symbolic protection (p. 45) and more abstract, isolated versions (see below), but afaik snow isn't mentioned as a medium here.
Lycanthia pp. 165-6; of the three characters in this excerpt, Luc and Gabrielle are werewolves and Christian is a n00b--
The marks in the mud filled Christian with irritation and loathing. He resented them, and said inconsequentially, "For such a religious community, they're remarkably careless. I never yet saw one of their damned crosses set straight."
"Nor would you," Gabrielle said behind them. She had come out on the terrace noiselessly. Standing by Christian, and gazing at the lopsided cruciform, she said, "Clearly, you don't know what that mark really is."
"Clearly, I don't. Just some superstitious sign against the two of you?"
"And against you, monsieur," she snapped.
"I?" Christian smiled. He recalled his excursions to the village, the closed doors, the solitary watching figures, the people who hurried by out of the cemetery. The carved seat in the church.
Luc danced back. "Look," he joined thumb and forefinger in the sideways cross Christian had seen daubed on the post by the inn. "That's no holy symbol, my friend."
"Then enlighten me."
Gabrielle glared at him. "You are in the forest with no weapon and no chance of help. A wolf leaps at your throat. Consider, life is more important than anything. Your instinct will sacrifice any portion of you to retain life, to protect the vital spot. What do you do?"
Very slowly, reconstructing, Christian raised his left arm and moved it over his neck, pressing as closely as was possible to obscure the windpipe. The gesture was awkward to achieve and to maintain, yet once he had done it.
"Last time," said Luc, "he used the other hand to strike out at me."
"This time," said Gabrielle, "he must forget his bond with the wolves. Forget your instinct, Christian, which told you we wouldn't harm you. You must insert as much of yourself between your life and the teeth of the wolf as possible."
As slowly as before, Christian raised his right arm and crossed it above the left.
"A difficult stance," he said, "and one which leaves the rest of the body unprotected."
"Instinct," she said. "Only that. The beast goes for the throat. You must protect the throat." She nodded now with smug briskness. "The sign you are making now with your arms is the sign you mistook for a cross. The sign they hammer up on the sides of their houses, paint on walls, create with two bones, or their fingers. Not the crucifix. The Lysinthe."
I can certainly see how the concept and the shape of the throat-protecting gesture could be borrowed as a counter-lycanthropic ward, even if not quite faithfullly described. I can also see how the name could be borrowed as well, and the two passages have relatively little direct textual overlap.
However, the underlying *structure* of both passages remains the same: some n00b thinks the X-shape is some sort of cross, but is corrected by werewolf snark and the suggestion of the "You're alone in the woods without even Granny's basket to protect you" scenario, causing the n00b to produce the parent gesture by instinctively throwing up his arms. (He shouldn't've eaten them in the first place.)
Considering that none of the werewolves run away howling when the arm-gesture is made in person, it doesn't seem to be particularly effective magic, pace Lupin; it doesn't explain why the werewolves themselves couldn't demonstrate it as a more succinct explanation, or simply say, "It's the same shape as what people's arms do when they're protecting their throats from a wolf." However, I consider this more of a head-scratching "This doesn't feel quite right" unease than the startled "Hey!" of noticing direct textual overlaps as with the previous post about DS14.