I don’t know what else has been said, but ten isn’t too young for stroppy moods. Hormones can start changing earlier than that. I knew girls who started their periods at that age, and that was 50 years ago. With the onset of the pill etc, it’s brought it even earlier for girls/ women in subsequent generations.
Try not to worry. Be yourself, and she’ll most likely come back. Just enjoy her sister for now.
How to Keep Living at Home Longer



