Wow, aren't family secrets something? I couldn't believe that my aunts and cousins feared my mother's wrath so much that they kept the truth about my Dad from me until her brain was so addled with dementia that it didn't matter anymore. It was really sad that their fear of her kept me from knowing my Dad for so long.
You and your sis have the right idea - work on building your own relationship. Does she have a copy of her birth certificate? Would it tell the truth, or was it falsified? I know that falsifying birth certificates was fairly common back then. Mine doesn't have my biological father's name on it.
Good luck with finding out your family genealogy. Let us know if you find out any more juicy tidbits - this stuff is interesting.
BTW - Congrats on the caffeine-free success. I haven't drank a soda in a long, long time - never cared for the stuff. However, nothing would ever, ever convince me to give up my morning cup of coffee. That is one vice that I will continue to have until they pry the coffee mug from my cold, dead hand - LOL.
Hugs & Happy Holidays,