Your piece is a little bit confusing at the beginning in understanding who the suspect was and who was the current/past boyfriend of which twin.
But, I reread it and I think I got it now. It's a crazy story nonetheless. But, I know patterns are extremely common with twins. Suicide, murder, eating disorders, abuse, etc., it's not just due to genetics but sometimes even due to the fact that the sibling feels like they are not fully themselves unless they copy what the twin is doing. They can also be EXTREMELY competitive in order to stand out.
In this case, I would guess that Edith was always attracted to the boyfriend and was either extremely naive about her sister's murder or was so competitive she didn't care in the end.