If a boy see that his sister doesn't have any clothes on is it courteous to just stand there and look at her?

Staring is very discourteous. If she hasn't seen you, just keep on going. If she does see you, then a simple "Hi, Sis" and keep on going.

If she's coming out of the bathroom after a shower, she knows that she is taking a chance that you or one of her brothers is out there and she's taking a definite chance that you will see her without anything on. So she should not later complain that you are a pervert if you see her out of the corner of your eye or if you should get a real good look at her from head to foot. You can get a good look at her without standing there and staring. If she really cared whether she got seen or not she wouldn't go naked.

No, it's courteous to look away. Staring at her makes you a pervert.

Only if you are looking into her eyes, not her breasts! (Respect is key and FUNdamental in this situation.)