When my sisters and I, along with a couple of cousins, were young teenagers there was a bit of rivalry and bickering at times - it was just the usual crappy, hormonal, teenager years that bring on petty jealousies etc.
But by the time we were reaching late teens/20 it really didn't happen after that. But one of my close friends had quite a lot of problems with rivalry in her family from childhood, although it didn't involve her the most. To be honest I blamed her parents for that. Anytime I was around, the oldest and the youngest child were always noticeably favoured over the the middle four kids. My friend was the the youngest of the middle four and even today she recalls not getting much attention from her parents even when she achieved at school. I will add that she is a wonderful parent with four grown up children now - and no sibling rivalry.