I entirely disagree. I will get downvoted for this but I think nobody did anything wrong here. I think OP is reading into this situation because of her perception of the SIL's character, I think anyone in OP's shoes would have felt the same, but I also think anyone in her SIL's shoes would have felt the same as her.
I don't see it as the SIL demanding anything. She had her first baby - she has never had anything bigger and more exciting happen in her life and, in her eyes, her family wasn't there excited with her. It wasn't about the wedding. It wasn't about ruining OP's day. She was just upset that no one was just as excited as she was about the birth of her first child. If she had chosen to be induced this day specifically then I would completely agree with OP. But look at it from her eyes, she has a baby, her first baby and the first grandchild, everyone rushes over says "yeah okay that's cute and all but we have a wedding" and then just leaves. Here she is on the most exciting day of her life and no one acts like they give a shit. Yeah there was a wedding, but maybe just maybe she wanted her family to be as excited for her as she was.
But that's not the case. Its simply a case of coincidentally poor timing and parents who were being stretched in two directions. I mean, dads, everyone is making them the bad guy - their son got married AND their daughter had her first baby! They made it for the first part of the reception, and then they left to be with the new baby. In my eyes, that is totally reasonable and acceptable. But everyone is making them out to be monsters.
And to play devils advocate to everyone saying "well the baby will be there forever but you only get married once" - the marriage will be there forever and you only have your first child/grandchild born once. It's 50/50 and neither is more important than the other.
Hive mind is really taking over in jumping on the "Yeah OP, they are fuckers and you SHOULDNT send them a fruitcake at Christmas!" Which is totally bs. Because she didn't have the baby intentionally to spite OPs wedding.