My Husband and his sister have a weird relationship and it makes me feel awkward

So when we first got married, his sister would call my husband “baby” and I thought that was the weirdest thing ever. I asked him about it and he was so upset I would even insinuate anything about it, that it was just an endearing word used out of love. I continued to hear her call him that for some time and I made a fuss about it, because it was just plain weird to hear that word be used especially when we were newly married and that was my word I used for him. It was an ugly fight between him and I about it, but after his sister never used that word ever again on him. Fast forward now 5 years later and I just can’t keep my mouth shut anymore about their weird chemistry. That last fight was bad and I kept my mouth shut for 5 years about, but whenever they talk to each other (which is a lot, they call everyday) it literally sounds like he’s snuggling with his phone whispering sweet nothings with her and them both giggling and blushing with whatever they’re talking about. It’s uncomfortable because he’ll purposely speak even softer and lower so I can’t even hear even if I’m in the same room. He only does this with this one sister, and he has 2 other sisters who he talks normally with, sounds like with them how a normal brother sister conversation would sound like, but this one sister is completely weird, if I was a third person not even married to my husband, I would think he’s not talking to his sister but to his wife. When we traveled together once, him her and I, everyone on the plane, to the security checkpoints mistakened her as his wife, that’s how close they both were acting towards each other. I stayed quiet all these years, picked my battles because he got real ugly over it when I even tried to question why him and his sister seem so weird with each other, but she just recently gave birth to her first child, and now she’s calling my husband asking him all sorts of questions and private details she should be asking and confiding in her husband, not to mention my husband and I don’t even have children yet. What could my husband help her with in that department? Anyways I made a comment to him that I find it very odd and weird that they talk the way that they do, why do you guys talk to softly and tenderly with each other? It sounds like I’m intruding between a couple. And he got so upset over me saying I’m jealous over what relationship they have as a brother and sister, and basically told me to shut my trap and leave that he’s done with the relationship. So what, am I supposed to just feel uncomfortable and see all these things and feel a certain way and not comment on it? I lost count how many times my husband mistakenly called me by his sisters name, in inappropriate times too. I find it very odd. He’s never mistakenly called me by any of his other 2 sisters. I’m not jealous, I’m weirded out, completely. Currently we’re in a bad fight over this again because I made a comment about it and he’s “done with this marriage” because I’ve insinuated such a dirty thing. I’m not aggressively accusing him about it or anything either, at first he laughed about it, and so did I, all I keep saying is that I’m genuinely weirded out, it makes me uncomfortable, and now he’s literally raging over the fact I said this.