Is this going too far?

First of all: I am 12 and therefore still a child myself!

Hey!

I have a little brother (1 year old, he was actually unplanned) and I see him every 2 weeks on the weekend (because he is from my father and stepmother and I live with my mother) and always for half of the holidays (for example, I am staying with my dad for another 2 weeks).
I love my brother more than anything, and he means a lot to me. I really enjoy taking care of him, but I wanted to ask if this is starting to go too far with my father and stepmother.

I feel like a second mother to him. But first, let me tell you:

I am always the first one (I have 2 older brothers) to be asked if I can babysit and I am always expected to say yes.
Recently, I said no because I was texting with my crush. The question immediately came: "Is xy more important to you than your little brother?" Of course not, but I wanted a few minutes to myself. The result: I had to take care of him after all.

It's often handed to me in the morning with the phrase, "Could you take care of xy for a bit?" It's usually a few hours. Recently, for example, I came out of my room and it was handed to me right away. I'd only been awake for 10 minutes and wanted to get ready. That wasn't the first time.

I also often get yelled at for "mistakes" (you can't really call them mistakes because he still has a lot to learn) that he makes. Here's an example: Yesterday we went swimming in a lake, and my brother put his feet in the water, even though he was still wearing his normal clothes (shorts, t-shirt). I was supposed to pull him out for a moment, no problem. I then took him by the hand to help him out of the water, and he sat down halfway, so his pants got wet in one place. I then had to listen to my dad scold me, which was completely unnecessary because his pants were dry anyway, and because he could have just done it himself at that moment (he was just standing there at the time).

I also always feel responsible when he gets hurt or something.

I'm not doing so well mentally in general and I'm in therapy, but it's really stressing me out because I feel as if I were already a mother.

I spend hours taking care of him every day and it makes me so tired and weak.

So is this going too far for my father and stepmother?

Please refrain from unnecessary things immediately… Thank you!