In The One Where No One's Ready, the gang checks the answering machine and finds a message from Rachel's sister. She asks if Rachel ever slept with some guy because last time he shouted 2-for-2!

Wait a minute, 2-for-2?, there are 3 Green girls, not 2. He should have yelled 2-for-3.

Hmmm.......

Thats true, but he could have made a mistake if it were the real world. Since he's just sleeping around with them he may not know they had another sister.

The guy was only referring to Rachel and Amy because all three of them were in high school at the same time, while Jill, being much younger, was probably in middle school at the most.