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The way you make a character sound condescending it to have them say condescending things. That's it. That's the whole of the recipe.

But I suppose you knew that. I suppose you must have tried that and had a hard time pulling it off convincingly. 

So here is a guess as to why that might be. I'm guessing, because writers get stuck like this all the time, that what you really want in this scene is to have your main character be condescended to. You are really only interested in their arc at this point. It is all about having this thing happen to them so that they can react the way you want them to. 

So why is this hard? Because it concerns the conduct of the other character. And if that other character appears in more than just this one scene, then they have an arc themselves and if their actions don't make sense in terms of that arc, and in terms of their established character profile, then it is just going to sound wrong, no matter what words you put in their mouth. 

And interaction between two characters, in other words, has to make sense in terms of the arcs of both characters. If it does, you will generally not have much trouble writing the scene. If it does not, nothing you can do in the scene itself is going to fix it. This is why writers have to rewrite their stories many many times. It is hard to get all this stuff to line up convincingly. 

Of course, if you need your MC to be condescended to at this point and the sister does not turn out to be at the point in her arc where she would do this, you can always invent someone else to do the condescending. The great thing about minor characters is that you can invent them on the spot to react to your MC in the required manner and then conveniently forget about them for the rest of the novel. You don't need to align their arc, you just need to paint a convincing portrait of them in the moment.