It sounds to me as if she has already chosen, Marie. I'm so sorry, I cannot imagine how frightening this is for you.
You cannot control her or her boyfriend any more than you could have controlled your XAH, or what happened to your middle daughter after giving birth, or the heart attack. The best you can do is protect yourself, as you other daughter has done by limiting her children's time with their aunt. That is not bullying, it's just being smart.
You have let her know you care about what happens to her. She is an adult with the freedom and ability to make her own decisions about her life. Just because right now she is willing to accept the circumstances, does not mean she always will be. It is important that she knows that if and when she needs you, she will never have to hear, "I told you so."
Big huge polar bear hugs to you, Mama Marie.