Most Helpful Guy
You don't make a point of SHOWING that you are ignoring him. You simply ignore him. Best way to describe it would be to say, let your attention turn elsewhere. As if whatever he is saying just isn't particularly interesting. You don't sigh, you don't roll your eyes, you just find something else to pay attention to.
This also does not mean pretend he doesn't exist. It just means that when your sibling is doing is "superiority" thing, you look for something else to occupy your time.
It takes time and patience but this will work best.
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