"I think it's noble as a grape the way you look after River, but she ain't my sister."

I agree, I always thought it was Matty but with parents who called their strapping young son 'Jayne' it could still be a younger sister.
For some reason I always thought he was from a large family. All with their gender non-specific names.
Oh dear. Now look what you two did.

What if he had a large family who's parents felt the obligation to name their children after their relatives. By the time little Jayne came along they only had the one maiden aunt left and the promise was wrangled long before the due date. Who would have guessed that a daughter was next? Aunty Jayne certainly wasn't leaving it up to chance.