I have a sibling, and we trained in the same group in Level 5 (her) -8 (me) on Saturdays, and we had a mental competition. We never talked about it, but now we admit; when one girl got a skill, the other one tried their hardest to get it too. Once, I came first in a competition, and her team had come third, and she was really upset. We weren't even up against each other, but in our heads, we were. I had gotten the better result, and I was so excited, and she kept trying to downplay the victory. She still remembers it, and says she cried because she felt she didn't do as well. And I accused her of not caring about my result.
We were 8 and 12.
The best advice to any parents struggling with that sibling rivalry is:
be over the moon with excitement for both children. That means they both feel happy with what they did, and they don't think about the others. I was sick if this feud eventually, so I decided to compliment my sister as much as possible.
'Your straddle jumps are beautiful, Anna!'
'Great job on your aerial, Anna.'
Then, I no longer felt I was competing with her, and she at least on the outside didn't appear to compete with me anymore.