Is AUTHOR wrong in the post ? POST : First time post here, and I'm writing on mobile, so yee.

Cast:
M; Mom.
D; Dad.
BS; Big Sister.
Me: Mary Poppins.

So a bit of background, I share a room with my 18-year old, still not moved out sister. She's almost never home so I usually have the room to myself.

As you all know, it's mother's day. About 10 minutes ago, while we were on a bus, M got a text from BS saying she wasn't coming home today.

M was slightly upset - it's mother's day, after all. She could've come home for even 5 minutes.

D said it's normal for her, and she only thinks about herself. 

Also, she's not even out of town. She just wants to stay at her friend's house literally every night.

I was pretty annoyed, but then again, BS does this alot. I get it, she's 18 now, she can do what she likes; but she didn't even say a simple, 'Happy mother's day,' to her.

I kind of want to contact her about it but I know she'll get all defensive.

AITA for getting mad at her?