I don't think it's too much at all! I actually really like it— I love how it shows how the girl wants to be and how her sister sees her in a very concrete way. I really hope you do end up writing it— this is a book I'd be very interested in reading! :^)
My name's Ben.
In short, no, not at all. The only way this is going too far is if you think it's your moral duty to portray bullies/members of the dominant race/etc as cardboard cutout villains.
That's not your duty and it's not true. Everybody should be humanized in most stories.
Timidity is about the worst trait a writer can have. Go forth! 
This is something that might inspire you for the girl's positive experience nearly drowning.
edited 30th Apr '16 7:07:28 PM by garridob
Great men are almost never good men, they say. One wonders what philosopher of the good would value the impotence of his disciples.
Shouldn't the aliens stop as soon as they're told about humans not being able to breathe underwater nearly as well?
How do you want your audience to feel? I personally feel really scared, because the aliens innocently not knowing about human biology makes it even scarier.
edited 30th Apr '16 11:51:43 PM by hellomoto
Grr... <3
I don't know the tone of your book, but that scene would work in almost anything. It lends a certain amount of realness and danger to the world. It establishes everything you want it to. It can be written multiple ways. It gives your characters something to talk about later. None of these things is a downside usually.
Okay, for the novel I'm currently batting around, there's a scene where the human protagonist goes skinny-dipping in a pond with her older adopted sister and her sister's friends, who are all aliens. At one point, the friends decide to show the protagonist, who has some deep-seated issues with belonging (because her adopted family, and almost everyone else she knows, is an alien), that she's worthy of being in their circle... by picking her up and tossing her in the pond, and then playfully thwarting her attempts to surface again just long enough that she nearly drowns (they can breathe underwater up to a certain point, so they don't realize that humans need to surface more quickly), as a sort of Initiation Ceremony. When she manages to surface, the other girls are delighted and take turns embracing her and kissing her, welcoming her to their clique. The friends are obviously happy she survived, the protagonist herself is ultimately happy with all this attention, but then the adopted older sister gets angry because her idiot friends nearly killed her sister, ending the scene on a bittersweet note.
There are several things I'm going for with this scene. First is to show that this is a very different world, with different mores. Second is to show how the protagonist's perception of her place in this world is at odds with how other see her place in it (she thinks she's an outsider, but these girls see her as one of them.) And third is to show one of the dynamics at work between her and her sister - her adopted big sister loves and cares for her, but is also one of the driving forces for why she feels like an outsider, because her big sister constantly reminds her that she's only a human on a world where humans are a minority.
But does it go too far?
edited 30th Apr '16 2:00:47 PM by StrixObscuro
By now, it should be clear to all except the most dense of us that sheep are secretly conspiring to kill us all and steal our pants.