Do you think this is odd for siblings to be doing?

So my friend lives in a really let's say.. liberal/progressive family. She has 2 sisters and 2 brothers. the sisters are aged 11,12 and she's 15. her brothers are 8 and 14. so, anyways, she and her siblings like change in front of each other! if they shower or something theyll just walk down the hall in their underwear. I've never seen anything ual go on but i was wondering others opinions on it! don't get me wrong, I love my friend to DEATH and wouldn't judge her. it's just interesting...