How on Earth hasn't the little sister noticed anything suspicious about the big bro's behavior (or his smell, huh)? Wasn't she the jealous type? Maybe her airheadedness made the sea trip capture too much of her attention.
@Dadidu I'd take it as strategy. The princess is currently feeling too tied to her position and wouldn't allow anything more, hopefully because she wouldn't want to endanger the MC (as if anything could be a danger to that god of war). She doesn't seem like a person who would feel something is beneath her station, so that can hardly be the reason. Friend is something the princess can accept for the time being, so he sticks to that. At least friends help each other, so it's close enough for the MC to be convenient.