I don't think the memories have anything to do with an adoptive mother, Leia remembers images and feelings. IMAGES and FEELINGS. These are things which are directly connected to the Force, Yoda even says so. Padme died in sadness, and in beauty, I believe Leia just had a momentary connection with her mother through the Force. And since it's such a tragic moment, it's stayed with her.
There's also the point that Leia's eyes are open when she's born and Luke's are closed (I can't confirm this but many people are saying it's so).
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