I don't want to attend my sisters third baby shower.
Why don't you want to attend the shower?
I'd rather spend time with my friends than waste my time with her for the third time.
I can see why going to a third baby shower could be annoying but she's your sister and you should support her. I'm sure she loves having you around and its a great excuse to hang out with you. If the shoe was on the other foot, I'm sure you would want her to be there for you as well.
It doesn't matter, because I am not her real sister. Besides, I'm busy this weekend and not interested in the whole shower thing.
Whether this is your sister's first or third baby, you are family and she could use your support at the shower. Let go of your personal desires and make the sacrifice to be there for her.