Hi Kat12, I noticed your post from a week ago. Like you I am part of the vast Mills clan, descended from Alexander and Emily through their daughter Emily. I recently asked the Priaulx Library in Guernsey to complete some research for me on Mary Eustace, Alexander's mother and got back the following information:
Civil death register.
Date of death:        17 September 1868
Name:                   Mary Eustas (widow of Alex Mills).
Age at death:         80 years
Father:                  Thomas Eustas
Address at death:   Rue Saumarez, St Peter Port.
Cause of death:      Decadence naturelle (Natural causes). 
 
My next step will be to ask the Priaulx Library to research the Eustace/Eustas family further, so we can find out the name of Mary’s mother, and Mary’s siblings etc. 
 
I see on Rootsweb someone has posted a family tree for Samuel, another of Alexander and Emily's children and I have possibly found their son Charles; the only one I don't have any information on is Thomas, who is possibly the twin of my great great grandmother Emily. I would like to start an informal e-mail network of Mills descendents (if there isn't one already). As this is your first post you may know you need two more before you can send a personal message in which case, if you wanted to, you could be sent details of how to join the Mills forum.   
Regards,
Craig.