BPD caused a family feud?

My brother is like my twin. I will do anything for his happiness. He and his wife were having marital problems to a point where it threatened the welfare of my niece. So I researched and observed for months before deciding to myself, no one else, that my SIL has BPD. All the classic symptoms were there. She was diagnosed with Bipolar Disprder but later found out that was the wrong diagnosis. I don't know if they told her what she had instead and I REALLY want to tell her my suspicion of her having BPD, but it wasn't until I spoke to her about my own depress/anti anxiety meds that she finally decided to try them after hearing my own stories about my severe depression and how meds have changed my life for the better. She has thanked me several times after for it when the meds began to take affect. It by no means "cured" her it just eased her anxiety, she still presents symptoms of BPD which is troubling but it's not as severe as before. I still want to mention BPD to them, but my brother's and her relationship developed a little evenly now, but now my dilemma is the fact that I have a niece from my brother and a nephew from my older sister. My family has always been about tight relationships with each other, never turn your back on family, stuff like that. Because my SILs first impression with my family sank lower than the titanic. They have never gotten over how "crazy" she was back then. I have told them that my brother and I have helped her see the reality of her actions and have gotten her a little help, but to no avail. Now my brother rarely gets to see our nephew and my sister rarely gets to see our niece. And I'm stuck in the middle. No one has anything nice to say about each other, I try to put my two cents in to mend the peace but no one will see past my SIL BPD behavior, and she won't see past my family's negative reaction toward her BPD behavior. How can I help mend my family's bond with each other? My brother and I are still as close as ever and I'm trying to get him to help me pick up our broken family but he still walks eggshells around my SIL so he might not be as much an asset as I had hoped he would be. I can't take this family feud thing anymore and I seriously don't want to wait for the day my niece and nephew are older and come to me with big tears on their little faces and ask me "Why doesn't auntie/uncle ever come to my birthday?"