I think you probably need more information to be certain about your niece in this specific instance. I agree with the others- fever or absence of fever means nothing. My son rarely runs a fever. My sister never does. Some kids are put on antibiotics quickly because they have histories of chronic infections and their doctors know they won't overcome it on their own. My sister is put on antibiotics for the slightest bit of sinus infection starting, because she has other health issues and the sinus infections won't go away on their own.
But Yes, I agree with you, antiobiotics can be overprescribed. It is hard to find a doctor that strikes a good balance. Some are adamant about waiting it out for a while to the detriment of the child, and some give them for everything and anything. I think it is terrible not to be able to trust your doctors. But so much of diagnosis is their best guess, and you have to just hope they take into account your child's history.
And no, you don't need an antibiotic for every infection. Your body is capable of fighting off some infections on its own. Before the invention of antibiotics, people didn't die or live their entire lives with a sinus infection. For the most part they fought them off. Sometimes they didn't and it progressed to something worse, but not always.
My son had a cold that turned into a sinus infection a few weeks ago. His doctor said that unless he got markedly worse, to wait 10 days before he would give antibiotics. And sure enough, it went away on its own. He has never been on antibiotics and he is still able to fight things off. Now if it was an ear infection, they would be quicker to treat it. But the body is an amazing thing if it hasn't had its immune system totally worn down. We treat colds the same way you do, and save the antibiotics for when they are really necessary. That way we won't have a resistance to them when we need them most. Antibiotics can absolutely be harmful.