When you're going to brag, you need to consider many things. Bragging is very difficult to do tastefully - and believe me, you want to do it tastefully!
First of all, try not to brag on yourself too much. Now I know that you're all thinking that I need to direct this to me, but I'm still working on this whole pride thing. Oops, I've gotten off subject.
As I was saying, your bragging needs to either be for the right reasons or very scarce. Bragging on yourself too much just makes you look pathetic, but every once in a while there are crazy relatives that want to know when you've done something cool.
Bragging on someone else is tricky. It seems as though it would always be ok, but you need to understand that not everyone has an amazing little sister - and you're not bragging on her to make people jealous, are you?
Here's an example. If you constantly rub it in people's faces that not only does your sister have an amazing personality and artistic skill, but she also won an award at a very popular Arts and Science fair they stop paying attention to how great your sister is and start comparing your life to theirs. In fact, if you keep harping on the fact that your sister is going to an awards ceremony that night, they might even start interrupting you and talking about how their little sister drew a cool picture of a dog one time. And if you mention that it was a photograph from an interesting perspective, they might just make a lame excuse and walk away.
And so, after much consideration, I have decided not to brag on my little sister too much. I can only say that she has recently done something great, and I'm very proud of her. But I'm sure that the picture your little sister drew of a dog was very nice. Even if it didn't win a major award.