If an adult in their early twenties moves out into a flat (that they pay rent for), is it acceptable for their younger sibling to use their room?
My youngest brother moved out in January and visited home this weekend. My youngest sister has been using the room as her study/work room. It looks lovely, she's put some nice pictures on the walls and some pretty pink curtains up.
My brother has gone absolutely mental with her and said she had no right and it's his room etc etc. My mum agreed 
But I think she's wrong! I don't see why my sister can't use it and think my mum and brother are in the wrong. It's not even his childhood bedroom, my parents only moved here a few years ago and he's always hated the house and the village as it's not where he grew up (hence why he moved out).
My brother is now also cross with me for siding with her but quite frankly he's acting like a baby.
He's been home once in 4 months and probably won't be back for another 4. Why can't she use it?!
Who is right? I'm going to text my brother and tell him to grow up, if anyone has a nicer way for
me to phrase this, suggestions welcome 
or is my brother