Ok first I'll say I think this story might be exciting but this 13 lines did very little to actually tell me what was going on. Your plot seems good, but the mechincs of actually getting the ideas across aren't jiving. I think if you reorder the sentences it gets instantly better. First let me break down the confusion factor. I'll take you through my thoughts step by step as I was reading, and I'll tell you what I was thinking as a reader. This might sound harsh, but please bear with me. I think there's a simple fix here:"The bath water was black."
So when I saw this I was like why on earth was it black? It's a great hook but then you never explain why. Is it dirty? Did she just roll in the mud or something?
"I could barely see my own reflection as I stared down at it. I realized that my hands were shaking-I felt good though, calm."
This confused me because I have no idea why your hands shaking has anything to do with the water being black. Maybe if you'd explained that already then I'd be OK with this. It's a another mystery thrown in but my brain can't handle two mysteries at once...at least when they are less than two sentences apart from one another...but then again, maybe that's just me My mind just can't make any mental leap as to what the heck is going on here. Of course suspense is OK, mystery is good, confusion is not good. Here my only guess as to what's happening but I'm pretty sure I'm wrong: (did she have an accident in tub? Is she sick or something? If that's what this is then a few more clues would help.)
"Mother came in. “You didn’t say they’d be people this time,” I whispered."
Another mystery. These dots simply don't connect in my head. Usually in a good hook it instantly throws us into the story, gives us enough details to give us something to ground us. Here, I've been tossed back and forth from she's been mud wrestling, to she's sick, to she pooped in the tub because she's shy? That can't be right...but that's all I'm coming up with here.
“Do you know how much work it took me to capture them and bring them all the way here, and the first thing you do is complain?"
This is a little better. Now I'm getting some idea that they are evil...but that's still about it. Why would they be capturing people? How does that have anything to do with a black tub...or shaking hands...ect. I get that the girl is nervous and doesn't want to go through with something but that's not enough of a frame work to keep me in the story. This sounds like it could be exciting, but you haven't given me enough detail to have a clue what they are talking about.
I have a brat of a daughter then.”
This is a good comment. Good characterization. Still I don't know what they are talking about. Even though the lines are well written as a reader I don't care about the dialog until I can at least guess where they are, and what they are referring to.
I could still feel them inside my body. Stirring in there, and trying to claw out. I can still see their faces. There was a woman with blonde hair and a man with brown hair. That is all I know, I didn’t even look them in the eye.
OK this totally threw off all of my guesses. She's being possessed? I think you waited too long to tell me this. I think this part should go first and then most of the problems with the confusion would be solved.
********************
So now...the light at the end of the tunnel:
Try this order -
"I could still feel them inside my body. [They stirred], trying to claw out. I [ cut - still] [saw - past tense] their faces. There was a woman with blonde hair and a man with brown hair. That is all I [could see - past tense]. [I couldn't bring myself -past tense] [to] look them in the eye."
Now, we instantly get that we are talking about someone being possessed. This is the perfect framework for adding the rest.
"I could barely see my own reflection [in the black water] as I stared down at it. [Grabbing my hands to steady them,] I realized that [they] were shaking [.] [Despite this,] I [was] calm."
Since the black water didn't really get explained I de-emphasized it. This way, it's just black. We don't really expect an explanation about it and I think that's what you were going for. Something you might want to explain though is why her hands were shaking. Normally people associate this with being nervous, so if she's calm...there's another reason. That reason should come soon after this statement or it just feels like a contradiction.
"Mother came in. “You didn’t say they’d be people this time,” I whispered."
She now this is really flowing. This is good in the new order. It makes so much more sense now. And when she says that, it still give us a mystery, but because we got that frame work going in the opening sentences we aren't protesting anymore.
“Do you know how much work it took me to capture them and bring them all the way here, and the first thing you do is complain?"
[Mother really knew how to put on the guilt trip. I crossed my arms pouting, protesting in silence.]
[She narrowed her eyes,] "I have a brat of a daughter then.”
So I didn't see the main character answer her mother's question, and if she doesn't we need to know why. We need to hear what's churning in that little head of her's, and it also helps to see a physical response from the mother.
***********So here it is all together now:
I could still feel them inside my body. [They stirred], trying to claw out. I [saw] their faces. There was a woman with blonde hair and a man with brown hair. That is all I [could see because of some reason I couldn't bring myself to] look them in the eye.
I could barely see my own reflection [in the black water] as I stared down at it. [Grabbing my hands to steady them,] I realized that [they] were shaking [.] [Despite this,] I [was] calm.
Mother came in.
“You didn’t say they’d be people this time,” I whispered.
[She sneered,]“Do you know how much work it took me to capture them and bring them all the way here, and the first thing you do is complain?"
[Mother really knew how to put on the guilt trip. I crossed my arms pouting, protesting in silence.]
[She narrowed her eyes,] "I have a brat of a daughter then.”
****
Hopefully all of that didn't offend you. It really was a slight adjustment, but I wanted you to see why. To sort of get inside the reader's head there.
OK, well good luck with this.