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Maybe your sister isn’t actually trying to sound Australian. She may just be susceptible to the crowd she’s with. I’ve noticed that I tend to temporarily “pick up” accents of whatever group I’m with. It’s not that I’m consciously trying to sound like them; it just happens.
FWIW, I tend to sound sort-of Australian on the rare occasions that I’ve been drinking a lot. I don’t know why, but my theory is that my brain is trying to articulate, but it’s thwarted by the alcohol. By coincidence, it comes out sounding vaguely Australian.
I met my first g/f’s family, and one of her sisters asked me if I’m Irish. Nope. Never been there. As far as I know I speak Standard American English. But there was “something” about my voice that led her to believe I had an accent. I have no idea why she thought I had an accent when I talk like everyone else around here.
At an airshow an Englishman thought I was from London. I’m guessing that I was trying to be overheard above the aircraft noise, and in an effort to make myself clear my syntax and inflection might have changed in the shouting.
A girl I knew from New Orleans thought I had a “surfer” accent. Cha! As if!
Once someone told me I sounded Welsh. 
I also have a tendency to “do voices”. (Peter Lorre is a favourite. I’ve recently come up with something that sounds a little like Jimmy Stewart, I’m told. I do a bizarre voice that is intended to sound like like an English thug. A German voice comes up once in a while. And of course, there’s the Pirate!) Maybe when people think I have an accent it’s just these “voices” influencing my speech? Or maybe it’s years of watching Monty Python and The Young Ones? Maybe I really do have some sort of accent of which I’m not aware, that sounds vaguely English? (No. I don’t.)
So maybe your sister is just “picking up” accents subconsciously.
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