"He didn't tell me what happened until now, as he didn't want to trigger me, and I thank him for that judgement. But really, who tells such stories to new parents??"
The same people who tell stories about miscarriages and babies born with deformities to pregnant people. It's hard to believe, but some of these people aren't being deliberately hurtful, sometimes it's just cluelessness. In the case of your SIL, it's probably wrapped up in her own issues of not getting pregnant. Now that you know, you can be prepared to bean dip if she brings this type of thing up again.
Congrats on the new baby.