Frankenstein: Sister, Cousin, Fiance?

"My children, my firmest hopes of future happiness were placed on the prospect of your union." So, the relationship between Victor Frankenstein and Elizabeth really bothers me. It might be just the time period and the style of writing, but I find the whole relationship really weird. First, the mother adopts a little girl, but teaches her son to call her cousin. Also, from the first moment she adopts her, she had the intentions of an "arranged marriage" between the two. The children grow up together as friends and playmates, but always with that undertone that there was always meant to be something more. As Victor grows older and goes to university, he still has that relationship with Elizabeth. The conversations and letters between the two have an intimate nature, but the letters and conversations between other characters also have that undertone, so the whole thing confuses me. I wonder what will happen between Victor and Elizabeth because we already know that Victor almost died chasing Jim (the monster- I named him) in the Arctic Circle and that he and Elizabeth were mourning the loss of William, but we do not know what happened in between.