If I buy a gun am I legally obligated to tell every household member?

no, but maybe morally...

No you are not. For the same reason you don't have to tell them when you buy a new hammer or drill set. Is it wise? No not really, accidents happen when people are ignorant of and unaware of a potentially dangerous situation. If you've got kids, buy a safe, I recommend one with biometric access so you can get to it quicker. But the reason children die from guns is ironically because of the anti gun mentality that is spread around. Using ignorance as gun control is as effective as teaching abstinence only to prevent pregnancies. It doesn't work. If a kid doesn't know about guns such as how to safely operate one, they are likely to kill or maim someone, probably themselves.

So yes, it's legal to own a firearm and not tell the others living in the house, but if you do, keep it locked up.

No, you don't have to tell anyone anything. If there are kids in the house then you should mention it to someone, but legally no.

Of course ... there may be people in the house that are not allowed ..

or simply dont want to be near guns

Not only are you obligated to tell them, it is your responsibility to teach them about gun safety and everything there is to know about guns,because its your firearm and you will be responsible for anybody getting injured or killed with your firearm,so its a big YES,tell them.