I don't think "the kick inside" means that there's actually a baby kicking inside her. And I don't think that the song is about her having sexual relations with her brother.
I think trhe kicking inside symbolises an urge to go out and explore the world, and that's why she has to leave him behind. When she says "your sister I was born" she doesn't mean they're blood relatives, but brother and sister in other ways.
I don't think "the kick inside" means that there's actually a baby kicking inside her. And I don't think that the song is about her having sexual relations with her brother. I think trhe kicking inside symbolises an urge to go out and explore the world, and that's why she has to leave him behind. When she says "your sister I was born" she doesn't mean they're blood relatives, but brother and sister in other ways.