The fact that they stayed with your mother and stepdad speaks volumes, they have no problem with them around their daughter, as your brother made clear when he told you he thought you were wrong to set limits with your mom and stepdad and that he would never do it. It doesn't mean that you don't have valid reasons to limit contact, just that they feel differently. Maybe they would change if their daughter was put in harm's way, but that's another story. The point is that your brother and SIL have taken sides, and that side is not yours.
You also said that your brother and SIL offended you about a year and a half ago, that you apologized and moved on, they never apologized or budged. Seems as if they don't feel they did anything wrong, and, therefore, saw no need to apologize. As a matter of fact, they may still be feeling you were wrong to call them on it, which would explain the "distance" you felt on their visit. Honestly, if you have moved on that incident isn't important any longer, it's done.
It truly sounds as if your brother and SIL came primarily to visit with your mom and stepdad, not to visit with you, or they would have contacted you themselves. And I believe your mom and stepdad were aware of how your brother and SIL felt, hence the awkwardness when you made suggestions for getting together. Plus the fact that you wanted to share and your SIL did not and didn't talk or open up to you tells me that perhaps the times together were not really wanted activities on their, or at least her part. Stepping aside and contenting yourself with the 2 "scheduled" times to get together might have been the preferred way to handle things.
And being one who advocates breastfeeding, (I nursed my son until he was 3) I have to say that the comment about how you were impressed that her sister breastfed for 2 years, when you KNEW your SIL thought it was ridiculous, strikes me as mean and calloused, kind of a "haha, told you so" comment, as if you were trying to put her in her place. Maybe you really were impressed and felt you were complimenting the sister, but it didn't come off that way.
I have a similar situation in my family, I brought something to my then future SIl and brother's attention, I was then uninvited to the wedding and we (SIL and me) have never been close. The night my father died earlier this year in February they were called back when it became obvious he was dying soon, and they arrived after he was gone. I hugged and talked with my brother (who has never had a problem with me) for a few minutes, and as he moved to someone else I told my SIL, "Let's end this now, I want another sister." and we cried in each other's arms. She was nice to me at the memorial and when they came to visit over the next couple of weeks, but was slightly cold at the funeral. We haven't spoken since, though my brother has been back to the house a few times. It is what it is, she is still upset that I called her out (nicely) on something years ago, frankly it is her loss, I'm a great person, as I'm sure you are.
You can't force anyone to want to spend time with you or your family. Unfortunately, sometime family hurts us more than others, simply because they are family, a "friend" we might just blow it off. I certainly wouldn't bring this stuff up, it will sound as if you are needy, and that you haven't got their message. Think about the things said to them a year and a half ago, and ask yourself how you would feel had roles been reversed and the things you said to them said to you. Sometimes, and I say this from experience, our words, no matter our intent, come back to bite us in the behind. We may not like it, but sad to say, we bring it on ourselves, we reap what we sow.
Keep at arm's length from your brother and SIL, and if they someday decide to be back in your life welcome them with open arms and be the better person. Oh! And be happy your niece loves her grandmother, your mom, so much, that's how it should be, regardless of if she's an alcoholic or not. Family is family, we love them in spite of their faults.
I truly hope that none of what I have said has offended you, but rather opened your eyes to what has happened, and help you to understand.