I think that’s a perfect example of what I’m thinking of. When you are younger, you may not even really know what your goals are. Or they might still be very abstract and far off in the future. Like I was open to eventually getting married and having kids someday, but certainly not in my early 20s. So unless you have two people who are very sure of their plans regarding marriage and/or kids I think it’s easy to settle into a sort of limbo relationship like your sister in-law. Like if she really wanted marriage, that conversation should have happened well before 12 years.
Some years ago, my partner and I crossed into the US from Canada with another couple. The customs officer asked “are you married?” and my partner said “Yes.” So did the guy in the other couple. Which was true in both cases. Except neither was married to their partner in the car. Then-partner and I now coming up on our 40th anniversary.
The first time I vacationed in St Martin with my gf my divorce was still in process. I was having a Bloody Mary and an espresso one morning… The waitress asked if my wife would be joining me. I told her I hoped not, as my wife didn’t even know I’d left the US.