Has your sister had a second opinion? And a third?
The medical community is notorious for recommending hysterectomies when other options do exist. I don't know enough about hysterectomies or your sister to make any judgements, but I think that multiple opinions are in order.
Good luck!
P.S. My mom had the same condition as your sister and went through a hysterectomy when I was 4 or so. She's not sure to this day whether it was necessary, but she doesn't really want to know. She'd rather believe it was, since it kept her from having children other than me. I guess I just don't want your sister to have any regrets. She might regret only getting one opinion, but I doubt she'd regret getting two or more. Maybe she already has, and this whole post is irrelevant. Good luck either way!