What do you think is the problem?

So its my sister's birthday and mom is going off, I don't know if she's upset with my dad or what it is. She's saying things like she wishes we weren't born and other things I feel isn't necessary. I don't know what pissed her off but I don't think it's necessary. My mom has always been like this, she says hurtful unnecessary things and she never apologizes. I don't know where it came from. I was the one cooking so it can't be because of that. My mom is always the one who ruins anything we have going on. It's like she's never happy. We do have something for her on her birthday so its not that.

I asked her what was the problem and she told me to shut up and mind my own business. Anytime she sees my sister and I talking she gets upset.