You are wondering how these changes will affect your niece. The answers depend on what is causing these changes. If you think of your body as an orchestra, the conductors are your hypothalamus and pituitary glands. They direct different glands such as the ovaries. This is just like a conductor directs the wind instruments or the string section. So what your niece will experience depends on where the problem shows up. Did it start at the conducting hypothalamus/pituitary glands, or is the problem at the ovaries?
Other questions include: how old is your niece and how quickly is she aging? An evaluation includes carefully listening to her story, examining her, performing some lab tests and an x-ray of her hands and wrists. Further imaging may be needed after that. One concern may be her height and how to make sure she will grow to her normal adult height.
She may not need treatment. If she goes through puberty slowly or it stops until the average time, then nothing may need to be done.
Sometimes children take medicine to halt puberty until they are older. When they stop the medicine, they will undergo normal puberty at the usual time.
Other children may need to take medicine to manage very high levels of hormones or they may need surgery to remove growths.
Another area of concern is how your niece may be feeling as her body changes. It can be challenging to look older but still feel like six years old on the inside. This can lead to teasing and feeling different from her peers.
Talk with her medical provider to find out more information. How fortunate your niece is to have a good and caring ally in you. Good luck.