I messed up but was right

I talked to my parents about my sister's boyfriend, concerns of cheating, abuse etc. They talked to her and she in turn stopped talking to me. This was 5 years ago. She recently broke up with that boyfriend from the reasons mentioned above and I am no way wanting to say I told you so, I just want my best friend back. I feel it's been more difficult because she is bipolar, she also tends to hold grudges just like our mom did. Any advice on what I can say in an email/ text that can get through to her without sounding condescending?