The time frames "currently" and "at the moment" can refer to a temporary situation, but they can also refer to a situation or action that might continue into the future.
Of the two, I would say that "at the moment" most often refers to a transient or ephemeral action/state, and yet, it most often takes present continuous.
But in some cases (with non-stative verbs) it simply cannot take simple present:
- Where are you going?
- At the moment, I go home (wrong)
- At the moment, I am going home.
"Currently", on the other hand, most often applies to actions or states that are continuing, and might continue indefinitely: :
- Who robbed the bank?
- Currently, we're looking into it.
yet it can reasonably take simple present:
- What inventory valuation method do you use?
- Currently, we use FIFO.
So, all I can say is that your prior notions about simple present vs. present progressive were much too constrained.
By the way, for what it's worth: "currently" extends a litlle into the past, whereas "at the moment" doesn't.
And, as you well know, one can say "I am going to school tomorrow." (But that's a different kettle of fish. Or maybe a horse of a different color. Or, more likely, a fish of a different color—namely, a red herring.)
The simple present is generally for actions or events that you do, or often do, or for discrete events that you repeat over time. The present progressive is generally for actions or events that are going on right now.
For clarity, let's replace "to see" with "to visit" because that's the implied meaning in this context. Right now, as you write, you aren't in the process of visiting your brother. You simply visit him from time to time. So the simple present tense is sufficient. However, your brother is in the process of doing his degree. When talking about your brother's situation, the present progressive is appropriate.
This is why the correct answer has "mixed" tenses, because you essentially have two different but connected sentences, each with its own subject and verb.
Other examples of mixed tenses:
My mother is cleaning the house, so I can't play video games.
His roommate is studying for exams, but Bill still has to practice his drumming.
The opposition is saying terrible things, but the candidate still fights on until election day.
Best Answer
Neither sentence is formed idiomatically, but (if rephrased) either tense is fine.
This means you don't like it when I am in engaged in the activity of sticking out my tongue. For instance, possibly it makes my face look strange.
This means you don't like it when I do stick out my tongue. It's not talking about the process as I'm in the middle of it necessarily, just the overall action itself. For instance, you might think it's rude.