"Return of THe Jedi AND the books clearly state that Leia's real mother was alive with her on alderan"
The novelizations are trumped by the films in many ways, such as Obi-wan being Owen Lars brother, and Vader deliberately planning on Palpatine being distracted by Luke.
Specifically, the novelization states:
Quote
He looked down at their intertwined fingers. "Leia…do you remember your mother? Your real mother?"
The question took her totally by surprise. She'd always felt so close to her adopted parents, it was as if they were her real parents. She almost never thought of her real mother-that was like a dream.
Yet now Luke's question made her start. Flashes from her infancy assaulted her-distorted visions of running…a beautiful woman…hiding in a trunk. The fragments suddenly threatened to flood her with emotion.
"Yes," she said, pausing to regain her composure. "Just a little bit. She died when I was very young."
"What do you remember?" he pressed. "Tell me."
"Just feelings, really…images." She wanted to let it slide, it was so out of the blue, so far from her immediate concerns…but somehow so loud inside, all of a sudden.
"Tell me," Luke repeated.
She felt surprised by his insistence, but decided to follow him with it, at least for the time being. She trusted him, even when he frightened her. "She was very beautiful," Leia remembered aloud. "Gentle and kind-but sad." She looked deeply into his eyes, seeking his intentions. "Why are you asking me this?"
Nothing here that has to be on Alderaan, and nothing that can't be a Force dream - something that Anakin had quite a few of, and Leia, being Force strong, would have them as well. As far as the film goes, Leia only states the last part.
Lucas has never been beholden to the books, so to say this is a contradiction doesn't hold up.