A second opinion will reassure your sister and her partner, regardless of the surgery she chooses...if she had a choice.
If the masses on your sister's ovary are as large as "oranges" then it is very likely the ovary is damaged. I had a single mass on my left ovary in 2004; it was a rapidly growing complex cyst and by the time of the surgery, the mass was described as the size of a grapefruit. The ovary was pushed out of it normal shape, and had bluish streaks in it, due to the pressure from the mass.
In addition to the damage to the ovary, the fallopian tube was damaged, and for the same reason.
Prior to my surgery, I had told my doctor to remove the ovary and fallopian tube along with the complex cyst. My doctor told me that I had made the correct decision as the damage from the cyst was significant. In a woman still in her reproductive years (as I was) a damaged fallopian tube might result in ectopic pregnancy.
She should speak to her doctors and see if the ovary can be saved. Although 1 ovary works pretty much as well as 2 when trying for children.
I have just had a 20cm mass removed from my right ovary and lost that ovary in the process. My gynae said she could not save it as the mass had obliterated any sign of a normal ovary. I still have my left one though so should all be ok if I want more kids (I don't anyway)
If your sister is worried about fertility, one can still have children with only 1 ovary. The remaining ovary "takes over" . So I wouldn't worry about that..
However, if she has any concerns, it would be wise to have a 2nd opinion . I'm sure others that have had the large cysts will reply. Mine was small and I'm past childbearing years .. Maybe the ovary can be saved?
Tell your sister good luck and keep us posted,
Sandy