No...Eilonwy's movie is based off some books that are in turn based off Welsh mythology.
Aurora's story is based off a french Fairy tale.
Their characters are not related and have nothing to do with each other.
"Sleeping Beauty is a 1959 American animated film produced by Walt Disney and based on the fairy tale "La Belle au bois dormant" by Charles Perrault. "
"The Black Cauldron is a 1985 American animated fantasy adventure film produced by Walt Disney Feature Animation. The 25th film in the Walt Disney Animated Classics, it is loosely based on the first two books in The Chronicles of Prydain by Lloyd Alexander, a series of five novels which in turn is based on Welsh mythology."
