It’s My Birthday!
As a child, celebrating a sibling’s birthday might be a strange experience. The other guy is hogging all of the limelight! That can’t be right! Then then, your own birthday may be just around the corner, and you’d be the one getting all the attention and presents.
Of course, if your mother wishes you a happy birthday on the same day as your sibling, that’s a gift much greater than anything your sister could possibly get. If you ask your mother, she’ll likely claim that she made a mistake and that she loves you all equally, but you already know the reality.