I doubt this is what actually happened. Perhaps your brother-in-law or sister left something out. Marriage is a Sacrament in Catholicism. As such, the Church does not recognize marriages outside of the faith.
But what I am still not getting is why the Catholic Church wouldn’t let my brother in law convert, given that at the time he had no option to marry in the church since it still recognized his first marriage that couldn’t be annulled which he’d have done in a New York minute were it possible?
I’m not asking why they wouldn’t marry them, I understand that part, but don’t get the refusal to allow the conversion if he really wanted to become Catholic? It’s not as if an untold number of born-Catholics and other Christians aren’t ‘living in sin.’ I’d think any church would welcome someone who seeks salvation there.
I’m almost sorry I asked, and I thank you for your civil response. It’s bothered me on and off for ten years now, and the similarity of circumstances between these two marital histories and conversions seemed to make it a viable question.