Unfortunately, I have lots of experience being the mother of a child who wets the bed. Fortunately, it's all in the past and we have completely conquered this problem. Here are my thoughts on the matter:
It is so important not to blame the child for bed-wetting. No one likes waking up wet, and she does not do this intentionally or out of laziness. Bed-wetting is not caused by laziness. Being punitive or scolding will NOT help reduce the problem, and will only damage the relationship between parent and child and cause anxiety about bedtime. Imagine if you felt you had to perform to a standard you had no idea how to reach--while you were asleep--or face the constant disapproval of the people you love best. Talk about pointless stress! You'd never want to go to sleep again!
If your niece has been dry at night consistently for at least six months in the past then this is considered secondary enuresis and she should be seen by a pediatrician to rule out bladder, urinary tract or kidney infections. Stress can play a part in secondary enuresis, but so can erratic schedules.
If she has never been consistently dry overnight, this is considered primary enuresis and is in the range of healthy normal for a four-year-old child. The hormone that makes the body produce less urine during long stretches of sleep than it does during waking hours needs time to regulate and develop properly in the growing child. The child's bladder and urinary tract need time to mature and the child also needs time to grow into recognizing the sensation of a full bladder. It's so normal. Primary enuresis is rarely caused by stress so parents do not need to blame themselves, although reducing stress in general will allow her to focus her energy on healthy learning and growing.
At this age, the treatment is time and gentle encouragement. I experimented with mattress and sheet protectors with my son for awhile, but resorted to Pull-ups ("nighttime underwear") for reliability and less hassle.
If bed-wetting is still an issue at age 6, you can consider using a bed-wetting alarm. There are a couple of medications to treat bed-wetting, but they are not permanent cures and the bed-wetting usually resumes when the meds stop. All they do is buy you some dryness while the body matures.
We got my son's Malem brand alarm from www.bedwettingstore.com. I also got an excellent book there called "Seven Steps to Nighttime Dryness." It is current and medically accurate and I highly recommend it. You could probably find it cheap on Amazon.com.
The alarm was not an instant fix and it required a real commitment because the parent *must* get up and help the child wake up. A typical time frame for lasting dryness is about three months. We kept a log so we could see progress--two accidents gradually turned into only one, that one accident started to come closer and closer to the morning, and then it got smaller and smaller as my son learned to get up right as his alarm went off, and then, finally, even before the alarm sounded. Small steps indicated real progress and keeping track meant we did not get discouraged during the process.
My pediatrician said alarm therapy is the only proven method of conquering primary enuresis, besides just waiting for the child to grow out of it. He also discouraged me from trying to parch my son into dryness by limiting drinks--dehydration is not the same as continence!
Teaching my son to double or even triple void (use the toilet multiple times before bed to ensure a truly empty bladder) also helped us a lot. You can teach you niece to lean forward while urinating on the toilet to encourage complete emptying.
Whatever you do or whatever the specifics of the situation are, please handle it gently, even though it is frustrating.
My sincerest best wishes!