Who Killed Elizabeth?

           I'm nearing the 300-page mark in The Luxe by Anna Godbersen. It's certainly a lengthy novel, but I'm enjoying it. However, one question that springs to mind is, who would gain the most by killing Elizabeth, the main character? The book begins with her 19th century funeral; the rest is a flashback to when she was alive. I believe it was murder, because she was last seen not alone, but with her friend Penelope, and drowned in the Hudson River. So, the question stands--whodunnit?
          In my mind, there are four suspects. The first is Diana, Elizabeth's younger sister. Diana fell in love with Henry Schoonmaker shortly after he became engaged to Elizabeth. And here's the kicker--Henry was in love with Diana too. Diana would have motive to murder her sister; then she could be with Henry. But I don't think she's guilty, because on page 295, "Diana went to her sister. She wrapped her arms around Elizabeth and pressed her close...they lay down to sleep, side by side for the first time since they were children..." I don't think Diana would show such a gesture to Elizabeth, if she was plotting to take her life.
         The second suspect is Henry, Elizabeth's fiance. The two were engaged when they were barely acquainted, and Henry was forced into it by his father. Elizabeth, already in love secretly, only accepted because Henry was wealthy, and her once-rich family was in secret financial crisis. Henry, as mentioned before, was in love with Diana, but could not act on it in public. What better solution to his problem than to kill Elizabeth? And who would question him when he had been betrothed to, and thought to be in love with,  the deceased?
         Another suspect is Lina, Elizabeth's maid. Lina had to work for Elizabeth for years, and they used to be good friends before Elizabeth broke it off. On top of that,  Elizabeth was secretly smitten with Will, her coachman--so was Lina--but Elizabeth won his heart. I think it's possible that Lina has enough pent-up anger and jealousy of Elizabeth to do the deed.
         The final suspect is Penelope, Elizabeth's best friend since childhood. Penelope has motive to murder Elizabeth because she had harbored a crush on Henry before he revealed his proposal to Elizabeth, and she had even had a brief affair with him. I think Penelope is the most likely killer, because when Penelope was with Elizabeth a while before her death, accepting an invitation to be her bridesmaid, she "tried not to smile too broadly. Already, she was being let in where she could do the most harm" (page 216). But by "harm,"does she mean humiliation, or something a little more grim? After all, Penelope was the last person seen with Elizabeth. So--what does this harm entail, and what is Penelope's definition of revenge?