AITA for not wanting to let my sister sleep in my bed?

I'm 16, and my little sister's 7. She's been having nightmares recently and is afraid of sleeping alone after she has one. It takes her a while to even get to sleep in the first place.

Almost every night she comes to my room and asks to stay with me after her dream. I understand how she feels (I've experienced frequent nightmares almost my whole life) so I let her stay even though I'm usually up most of the night.

For the past few days, though, it's gotten difficult. She wakes up really easily and it's hard for me to leave the room and come back (or do anything) without her hearing. When she's awake she likes to talk and usually wants to know what I'm doing.

Last night (around 1 in the morning) I was talking to a friend who was having a hard time. My sister came in and told me about her dream, and once again asked to sleep in my bed. I didn't want her in here this time, both because I wanted to be able to talk to my friend without distractions and also it just gets really annoying having a 7 year old in the room. I gave her one of my stuffed animals and told her she would be okay, and she said okay but a few minutes later my stepmom came into my room and told me off for being "cruel." She said that my sister's having a hard time and I need to help her out. I told her that I've let her sleep in my bed many times, but she didn't want to hear it.

She wanted to take my phone away but my dad convinced her not to. My stepmom still told my sister that I treated her unfairly and made me apologize. I get that my stepmom isn't a huge fan of me but aita?