Is Eilonwy the daughter of Aurora????

No...Eilonwy's movie is based off some বই 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 দ্বারা Walt ডিজনি and based on the fairy tale "La Belle au bois dormant" দ্বারা Charles Perrault. "
"The Black Cauldron is a 1985 American animated ফ্যান্টাসি adventure film produced দ্বারা Walt ডিজনি Feature Animation. The 25th film in the Walt ডিজনি Animated Classics, it is loosely based on the first two বই in The Chronicles of Prydain দ্বারা Lloyd Alexander, a series of five novels which in turn is based on Welsh mythology."

no

No, they just look a lot alike!

No.

Nah!

Nope, though I agree, she definitely looks like she could be XD

Now THAT is a ডিজনি theory in the making. They look a lot alike and both চলচ্চিত্র take place in England. I want to say yes.