Depends on the sister!
Mine (18 months younger)...
Funny
Random
Fucking needy
Rude
Lacks any sort of self awareness
Generous
Helpful
Bloody unhelpful
Loves a witter
Massively concerned with what other people think of her
Utterly struggles with listening to other people/taking criticism
Strong sense of justice/duty
Could start a row in an empty room
She's rung me three times today, wittered away, despite being reminded each time I am at work, appears to have forgotten and is surprised, though I have worked 8 hours every saturday for the last three years at least.
She asks my advice on utterly everything - then does the bloody opposite, I have told her she is a complete askhole multiple times.
Asks my advice then yells that I am telling her too many things.
I love her, she's my sister, I would also gladly fire her out of a cannon into outer space most days of the week.