Life's downers

Ok. So Keith and I should be pretty happy with where we are in life, right? I mean, we just bought our first home, our marriage, while not perfect (and if you say yours is, you're a lying liar), is pretty great; we have an awesome kid, are getting ready to place the order for Number Two. Our house is in chaos, but we knew it would be. Work is good. We both have good, solid jobs, good benefits, etc. We see FIL and stepMIL every weekend, and MIL maybe once a week. Alec is total Papa's boy; he loves his grandpa.
Life is good. We've worked our asses off to get to this point.
But there is still a pretty big bone of contention in our lives, and it's one that we both try not to speak of, but yet it's constantly underlying.
We have not spoken to Keith's sister or her husband since mid-January. We haven't seen our niece since then, either. Keith tried speaking to his sister a few weeks ago, but it ended poorly. Apparently, the problem is me. I'm the devil, simply because I speak my mind, and have my own opinions. Yes, I am opinionated. Yes, I want what's right for our family, and will also stop at nothing to attain a dream. So this makes me a horrible and manipulative person, who's turned her husband against his sister.
At this point, the original 'crime' isn't worth going into. At this point, it isn't worth going into what the problem escalated to. Because it's not pretty or pleasant. I was threatened, and then called a f@cking bitch, blamed for anything and everything, and told that an entire family hates me and that everyone "can see through me and my bull".
What I don't understand is how someone can be so narrow-minded that she will be downright abusive to anyone who doesn't agree with her. I don't understand how someone can be so spoiled and self-centered that it's deemed ok to spew nasty and hateful words towards someone, yet not expect to get it back.
This entire ordeal has eaten us alive. During a time that we should be happy, we know that we can't count on Keith's sister, and this kills a lot of it for us. The parents have to divide their time, plan around schedules, choose sides. Because Keith's dad was told on his birthday that if we were there, they would not be there. It's heartbreaking.
And then there's the constant calls made to the parents. My mother in law actually took it upon herself to tell me that "I need to learn my lesson about her daughter". This was in a league of it's own. What lesson is there to learn?
I can come up with a few different lessons learned. And they're not nice.
For those who know me, as well as those who don't; I pride myself on trying to make educated and informed decisions. I try to lead my child by example. I treat others as I would like to be treated. This is me. It works for our family, both individually and as a unit. If you disagree with me, I will always respect your position, and not hold it against you. For I am not you, and you are not me. I don't beat around the bush, and I don't mince words. I have my opinions, and so do you. The world would be a pretty boring place if we all agreed on everything, right?
And who knows? It's been known to happen that we learn something new from people on a daily basis.
This is my point.