Some women are just cunts, and pignasty can easily be another way to one-up their peers. Sounds like the sister-in-law just wants to divert all the possible baby attention away from the author and her deceased clump and back around to her and her still living clump. But she's also probably been an attention whore her whole life and it didn't just start with a positive pee stick.
I mean I'm sure the SIL's health scare was scary at the time, but she does not know "exactly" how the author feels. Having someone who has never experienced what you're going through sit there and say they know how you feel is so incredibly tone deaf. It'd be like if someone was talking about how much chemotherapy is making them vomit and someone else pipes up, "Ohh yeah I know EXACTLY what you mean! I drank too much last weekend and
almost threw up! Tee hee!" Maybe they're trying to relate, but they're failing at it.
I'm sure the sister-in-law is excited about her impending mistake, but it's insensitive to babble on endlessly about her pignasty in earshot of someone who just lost a wanted pregnancy. Plus I'm sure it's annoying the fucking shit out of everyone else. You can only be so interested in someone else's unborn loaf before you want to change the subject. I figure some talk about the pignasty is normal because she wants to be knocked up and she's happy about it. But read the room, lady.