Ugh, don't you just hate it when they don't take you serious like that? It's so humiliating and disrespectful. I'm sorry to hear your sister treats you like that. But the secret is in whether you believe in yourself when you say it. Minor fluctuations in the voice, and body language make ALL the difference.
It's a mystery why but apparently I radiate authority, so when I tell them to I need something, they won't even think about ridiculing me. Though, in the past I used to be an enormous push over. Was used by everyone emotionally, stepped on, humiliated, and was too naive to even recognize that this was bad behavior.
It think it's one of the reasons why I'm very organized and down to Earth today. :3