It can be normal.
Notice the word 'can'...depending on 'how' that rivalry is produced.
With friends of mine it has caused no amount of trouble.To the point of
one of the teen's being told...unless they stop! they will have to leave the family home.
I have seen it turn very ugly, with my friends children...and the family are not lacking for anything in life.(Which can cause issues).
If it is between a brother and sister, that's a concern, and maybe some outside help is needed.On the whole, it brings 'bitterness', unless it is stopped before it get's out-of-hand.That's the key, when they are young....it should be stopped, if it looks like it could very well continue.Those kinds of 'habits'. are not what family are all about.
It is healthy as long as people are not getting hurt in an aggressive or physical manner. People are born into families - they don't necessarily love it, so conflicts are bound to happen. It's a great process for strategizing and problem-solving, so it's acceptable.