My daughter didn't sleep through the night until she was 18 months old. I feel your sister's pain. What we ended up having to do was get her to fall asleep on her own. I had always nursed and/or rocked her to sleep. It was very hard to get her to go to sleep on her own at that age and it took us a long time since I didn't believe in letting her cry by herself until she fell asleep. I would put her in her crib and tell her that it was time to lie down. She wouldn't do it at first so I told her that mommy had to leave if she didn't lie down and then I would leave for a minute and come back and if she wasn't lying down I would do it again and leave for 2 minutes. I never left her for more than 4 minutes. She eventually learned that if she lied down I would stay with her. Once we got to that point I started leaving while she was still awake. There was a little protest, but not nearly as bad as before. In the meantime, we also had to take her pacifier away. She got a skin infection and the doctor said no pacifier. I think that actually helped too, since she didn't lose her pacifier in the middle of the night. That separation was much easier than I expected it to be.
I hope this helps. Tell your sister to hang in there. My daughter is 2 now and goes to sleep in a "big girl bed" all by herself and sleeps all night on her own. Every now and then she wakes up and needs mom or dad and we still go in but we calmly tell her that it is still night-night time and give her a hug and a kiss and maybe stay a couple minutes, then we leave and she goes back to sleep.
Good Luck,
D.