AITA for not sleeping on the floor?

I(18F) have a little cousin(15F) who's always been the spoiled one in the family. She feels entitled to everything I have just because I'm an only child and she has two brothers.

She always demands for the best, she gets the biggest piece of cake, the last glass of soda- whatever she wants, I never ask for anything and I don't argue cause she's smaller than me. This same day we didn't have food for dinner (We live in Venezuela. No food here.) they gave her a sandwich with two cheese and they only gave me bread.

Now, my cousins and I were staying at my grandma's house for the night. Me and my other cousin had to sleep on the floor of the living room cause everything else was taken. Him and I cleaned the big mess of a living room and placed two small mattresses on the floor. He immediately says "I'll take the most comfortable one". I was ok with that.

15F cousin (who lives next door to my grandma and has her own room) suddenly says "Oh you guys will sleep here? Ok, I'll take the other mattress".

That pissed me off a bit and said "that's where I'll sleep" but my grandma said she could put a blanket on the floor and I could sleep on it.

I became so mad, I cleaned that place, moved the mattress why did I need to sleep on the floor when she has her own bed? So I said "But she has her own bed, she can sleep comfortable in it." My family said I was rude, selfish and should act like the adult I am. My cousin didn't speak to me again and left to her room. I feel so bad cause I never demand anything and when I do they make me feel like an asshole.

AITA for not just accepting the blanket?