Well you just have to teach them that if they wake up they don't get anything out of it so they get used to getting themselves back to sleep. So no milk or coming into parent's bed or stimulation, just putting them back down in their cots if they're hanging on the bars, which is what mine does and calmly and quietly saying go to sleep. The first time they wake you go in after 5 mins, then lengthen it to 10, then 15 etc. I may have those times wrong, hope someone else can help. Or search on cc there have been recent threads.
She could have a look at this to see if any of it seems relevant (before buying the books!). Controlled crying worked for me during the day, but was impossible at night (too noisy for dd who is 2). Is she bf? At about the same age my ds was waking/feeding every two hours day and night for weeks. In the end I introduced formula just so that dh could take on some of the night feeds and give me a break. Once ds realised Mum's milk bar wasn't open 24 hours he stopped feeding so much. It marked the end of bf but at least I started getting more sleep. It may be a question of breaking the habit. I'm sure others will have some other ideas - your sister needs to find a method which fits with her. HTH.
When my ds wasn't sleeping I read Contended Baby Book (am I allowed to put this on here?) and it has some good tips. I found not letting them sleep after 2pm and getting into good night time routine helped - same time every night, bath, story etc. Also found a massage calmed him down. Still didn't have every night all the way through and this continued until 18 months which is hard when your working, we took it in turns getting up and just got through it.