UPDATE: My [19F] pregnant SIL [26F] is cold and short with me, but kind to everyone else

My original post is here!
So, I didn't get the chance to talk to my sister. When my parents came back from shopping with my sister, my mom started saying how she had the perfect onesie for her granddaughter. Before my SIL could start gushing, I asked what my mom meant. She asked if I hadn't heard I'd be having a niece, and I replied I hadn't. My sister said it was weird, because she'd asked her wife to tell me once they'd gotten the ultrasound. Her wife had zero excuse, and had the most deer-in-the-headlights reaction. There was something that just sort of snapped, and I started crying. I felt kind of spoiled and horrible for it, so I apologized, and went upstairs with the dogs. My parents came upstairs, and my mom went to go ask if I was okay, and my sister and her wife started shouting downstairs.
My mom basically said that my SIL does not like me, and that it wasn't really my fault- she's just bad with people she doesn't know, and took it out on me inappropriately. And yes, my SIL was still ****** about the wedding date thing, which just made it worse. I felt really ****ty and just... Just like really bad? Part of it wasn't my fault, but part of it was my fault. And apparently my sister knew about both issues beforehand, and had given my SIL directions to try and get along with me. They were still shouting downstairs, and I was kind of bordering on a meltdown, so I asked my mom if we could take the dogs for another walk. We went out the back, and when we came home, my sister had left the house to cool down, and her wife was upstairs.
I spent most of the rest of the day downstairs, until my sister came home. She had obviously been crying, and was in a bad mood. My mom took her into the kitchen to try and calm her down, so I kept sitting in the living room. I felt like a bratty little kid again. Like, I genuinely felt like I'd ****ed up my sister's marriage and probably ruined our relationship in the process too. Everything just felt awful, and when my sister came out of the kitchen, she didn't even look at me, just went storming back upstairs. Her wife started yelling first, and they were fighting for a while before it got quieter upstairs. My mom, dad, and I went out for dinner, and when we came home, my SIL was waiting for us.
My SIL apologized to me, saying she had been petty and rude, and that the wedding date thing had been a non-issue. I said it was okay if she was upset about the wedding thing, but I wish she had let me know. She said it wasn't that easy, since my sister would basically take a bullet for me, and it had caused them a big fight before their wedding. I said I had no idea, and that I was sorry to be the source of that tension, and I just wanted to be able to be a good sister-in-law myself, and be a good aunt. My SIL got angry, and said I had ruined the opportunity for the first one, and she wasn't banking on the second one. Then, she stormed upstairs, and she and my sister got into another big fight.
I felt awful. The bad feelings manifested physically, and I spent most of the night throwing up while my sister and her wife fought. My mom was really kind to stay up with me, and my dad tried to get my sister and her wife to get off each other's backs. I specifically heard my SIL shouting, "Who would you jump in front of a train for? Me, or her?" And my sister responded, without missing a beat, "My sister, no questions asked." They got quieter after that, and my SIL started up the screaming about an hour later saying, "I'd let all my siblings die for you."
My sister slept on the couch, and the two of us went out for breakfast in the morning. She basically said that after my SIL gave birth, she and my sister were going to separate. They would try counseling prior to the birth, and afterwards too, but if things weren't getting better, they'd be divorcing. I said that it shouldn't have gotten that far, and I was sorry for pushing the both of them. My sister said that my SIL does this to all my sister's friends, and they've been fighting often. But, she clarified that the way she treated me was a dealbreaker. She said it might've sounded sad, but I was her best friend from birth, and she wouldn't let me not feel welcome like that. I felt really guilty, and started crying again. She kept saying she wanted it this way, their relationship was broken as is, etc.
It's been tense and awkward ever since. My SIL won't look at me or speak to me, and I've given up. My sister has been by my side a lot, and has amped up the good sister behavior 10x. I feel like ****, everything is really terrible, and I'm sure I'm going to be making another post during the actual Christmas bull****. Not a great update, but there we are!
TL;DR: My sister and her wife are separating because of my SIL's behavior towards me.


Source.

Geez ....