Question: My sister and her husband married in a church 20 years ago and recently renewed their wedding vows. Their oldest daughter is 23, so they explained that they had been married in court earlier (which is true) but didn't provide specifics.
She asked me to query you: How much should she tell her children (now 23, 21, and 19)? They did not marry in a church earlier because they were both previously married and were waiting for their annulments to be approved. There were no children from their previous unions. But their first daughter was conceived out of wedlock, so if she knew the court date, she could figure out the truth on her own. In today's society, this is not a big deal. But they are very religious and have raised their children in the same manner.
Should they tell all or leave it in the past?
Answer: I'll speak directly to your sister, if you don't mind. My syntax is tortured enough.
If your past doesn't align with your teaching, edit your teaching - not your past.
Otherwise, the teaching will be a fraud either way; the only thing you'd control with the tell-or-don't-tell decision is whether you make your kids aware of the fraud.
Often, what drives these decisions is the likelihood your kids will discover it on their own, which makes some practical sense. However, it sounds as if the values you've taught your children are about principle, not pragmatism, so to keep your secret solely because you like your chances of getting away with it would be to double down on hypocrisy.
Presumably, the path of conscience here is clear, but if not, a word of caution: When it's possible to find out, kids usually do.
As for the best time to have the "Guess what! A bunch of the rules we've imposed are ones we've broken ourselves!" conversation, it's when your children are old enough to understand human frailties but still young enough not to sit there counting off the number of years you've pretended not to have any yourself. Think 8 to 10 years old, give or take, so you're latecomers to this particular party.
Your best chance of having that conversation now without completely disillusioning and/or infuriating your kids is to present your choices and rationales to them honestly:
You each chose to marry someone and then chose not to stay married. Say why.
You chose each other and then weren't entirely patient on the religious path to remarriage. Say why.
You then chose to raise your kids within a strict religious structure and without mentioning your forays outside of it. Say why.
You then decided to give them the full story as adults. Say why.
At every step you had facts, feelings, and beliefs to guide you through the profound challenges life presents. Whether you chose well or badly, each choice informed the person you are now. These are the mechanics of being human, and where the business of being honorable - or fallen - gets done.
If you can admit that, even now, you're still trying to get it right, with only a human's capacity to do so, truth and teaching will converge into an example immeasurably more useful than a fiction of living by rules.