Emotional safety is central to living well. If your sister would feel safest with a double mastectomy and if not doing so will leave her living with much more fear and limitations, given the family history, it may be the best choice for her to make. I have had to have one mastectomy in January for a large cancer, and am told that it is easier to get breasts to match if they are both reconstructed than if one is the original and one is reconstructed. I may well choose to have the other breast removed if I opt for reconstruction. Getting tested for genetic factors could also be worth doing to help decide whether to have one breast or both removed.
The Susan Komen page
http://ww5.komen.org/BreastCancer/De...umpectomy.html
has a section about deciding between Mastectomy and Lumpectomy. They say that one advantage of a Mastectomy is no radiotherapy is needed unless the lymph nodes are affected. The studies they look at (from 2005) show that there is a higher rate of local recurrence with a lumpectomy than with a mastectomy, but that because these patients are monitored, the 20% who had a local recurrence were then treated and the overall survival rates of both procedures were pretty much the same.
For myself, I feel safer having had a mastectomy and not having a higher risk of local recurrence that goes with a lumpectomy, and not having to have radiotherapy.
(These recurrence figures may well be changing though with herceptin and its family of drugs which help no doubt. I have not been able to find figures on local recurrence rates based on trials or research from the last couple of years. Does anyone know of any? )