I second what rmzbm wrote. We've used the "p&p" (though I've never heard it called that before!) for a little over a year. Are now ttc, but it worked for us. We only used it when I knew I was fertile, which really isn't that long in a cycle. I have at least one friend who has used p&p exclusively, and succesfully, for 7+ years. Honestly, the 3-5% effectiveness it looses to the pill and condoms, etc, is more than won back with the lack of hormonal tinkering (pill) or condom nastiness (I think we mightn't have found the right brand, or something). Admittedly, an unexpected pregnancy was never overly panic inducing for us though. Given your sister's fertility history, I would tend not to worry about it. Odd that she is uninterested in getting a real handle on the tcoyf methods, given that she is really motivated to avoid a pregnancy. I'm sure you did, but did you stress the fact that it's a great method of CONTRACEPTION, not just a method for achieving conception? Maybe she has it connected with you and your ttc journey instead of to contraception?
I love that you are so concerned about this - sisters are so special.