I, too,have endometriosis. The endometrium is formed within the uterus but in this case it escapes into the abdominal cavity. I've had a hysterectomy and gone thru menopause. When they did the hysterectomy they removed as much of the endometrium that they could. Some still adhered to organs but most of it was gone. Once the uterus is gone no more endometrium is made. The endometrium that is there will still bleed as long as the ovaries are making hormones but once menopause is complete there should be no more pain.
I did continue to have pain and an ultrasound showed that I either had a tumor or a cyst. During surgery they found that I had an inclusion cyst which was caused by scar tissue forming a pocket which filled with fluid. They removed that. I still have some pain when I jog or jump around because the scar tissue pulls on organs/nerves. I've eliminated that pain by finding what moves cause it. That means I no longer regularly jog or do jumping jacks.
I'm suggesting that since she's had a hysterectomy and induced menopause the pain is only indirectly related to the endometriosis. I would be sure to have the pain evaluated to be sure that something else, such as a tumor or cyst isn't causing the pain. If her ovaries are not producing hormones the endometrium itself should not be causing the pain.
I also had IBS (irritable bowel syndrome) which also causes pain. Have her doctors ruled out other possible cause for her pain?
If she has seen doctors about the pain, in addition to trying natural remedies I suggest she consult with a pain clinic. Biofeedback is one treatment that works for many people.
I sympathize with your sister. I had a whole lot of pain thruout the years too. I've been basically pain free for 15 years now and it is a relief.