I managed to find the Sweet Slumbers set on Tuesday, and it kind of made me think about Star Dream's origins, and why she's staying with Applejack. I think that Star Dreams is Twilight's future foal. Twilight was going to be busy for a few days, so she let her stay with AJ. Star Dreams has similar hair colours, and her coat is a lighter version of Twilight's coat. She's also a Unicorn too.
Thoughts?