Question for those who discovered late they were adopted....

I'm at a bit of a loss here - but not in a way that is hurting or damaging me. Hence why I just joined this forum, because I have been asked to submit an opinion on a future course of action, and while I have what I believe might be my final recommendation - I'm still going back and forth in my head on what I think the right thing to do is. And because this relates to my adult sibling - someone with whom I have a serious bond, for my entire life (I'm mid thirties) - I definitely want to push for what is ultimately best for them. (just to note - I am keeping genders, ages, and any details that could give it away completely out of my story - so forgive the blank holes. My sibling does not know - and as close as we are, I don't think they have a clue - and until such time as the parties in question decide what to do - I would rather eat steak knives than be responsible for letting this out until it's right, or even at all. Only in the extremely unlikely event that they have concerns they haven't relayed to me, and more unlikely event that they would find their way here) As I said - I am in my mid-thirties. My entire life - my view was that I grew up in a family with two children, that I was the youngest, and that I had an older sibling. We've always been close - as close as siblings could be - our entire lives. There was a story on how our parents met - and frankly, as we got older, we came to understand the story as a marriage born somewhat of necessity (i.e. our mother was pregnant with my sibling before they were married) - but as family stories go, it was pretty tame, and we've even joked about it. My parents are still married, and happy. So - we never had even the slightest moment where the story didn't seem right - or that there would be any reason to question anything. That being said - my sibling and I look nothing alike - but they do have enough features similarity for it to be explained as reflecting a different side of the family gene pool. I even have made jokes on it in the past - noting our lack of physical similarity and even making jokes that my sibling was the proverbial "milkman's baby" Not too long ago - my father visited for a period of time. We're pretty close as well (this is a tight-knit family - even though we all drive each other nuts occasionally) Without drawing this out - in our naturally in-depth conversations - my father revealed to me the last family secret: my sibling is not my father's child, and that he had in fact adopted them. And that the sibling I had, without even a moment of question, always thought of as my full-biological sibling is, in fact, my half-sibling. The struggle with this is NOT with me - my sibling is my blood, and there is no change in how I feel about that. The struggle is: I have been asked, frankly, for my opinion on disclosing this to my sibling. I've eliminated the "accidental disclosure" risk here, for a couple of reasons. For one - any paperwork that still exists unseen does not disclose the difference in paternity - as the father in question was gone months before the birth (though my father knew him - but does not have any idea where he might be, if he is still alive, etc). My sibling is my mothers child - so there is that. Furthermore - I was made aware that my fathers siblings were aware at the time, and accepted the course of concealing the fact. My grandparents, for example, took this secret to their grave - and never treated my sibling as anything other than their first prized grandchild their entire lives (I know this, because I was always kind of jealous). So the odds of one family member saying anything have, in my opinion, completely diminished over time to the point of nil. So if the decision is made to protect that secret - it could be done. I know I myself am perfectly willing to keep it until I go to my own grave. My thoughts are - my sibling is a few years older than I am, at a point in life where you have largely fully established who you are, where you came from, etc. And in ALL respects but one - genetics - this is absolutely true. I believe in distinguishing truth vs facts - the idea that family truth is what we make it to be, and generally, we do that for the best. So I believe my sibling has a life of truths that are no less valid than mine. But my flip side is this - and something I still can't answer: for those of you who discovered late in life that you were, in fact, adopted - retrospectively, after finding out, and getting through the turmoil that I would imagine this generated - are you glad you knew? Do you feel, after everything was settled, that you were given a better understanding - and the possibility that you have familial connections that have enriched you? In short - when I say I am not in favor of ever telling my sibling, because I don't want to confuse truth with simple facts - am I being selfish, protective of my sibling - or would I be encouraging the denial of opportunity? If anyone wants to comment - thoughts are appreciated.