Ive had unprotected sex with 2 girls at the same time at a party. A lot of the other guys there had condoms on, but there was not enough condoms to go round.

Yes.
It's not like you get oregnant every time you have unprotected sex, but risk is still too high to try that.
You wont't die every time you hit the wall with your car going 70 miles/hour but.... it's very risky.

But...
She's your half sister :o

To the OP, we use the pull out method most of the time and the rest we used condoms and no pregnancy..

I have..but it was a spur of the moment thing. The pull out method is simple but not easy lol. Using a condom is usually the way to go...it does loose some of the feeling and intensity though.

Plenty of times