There are two categories of Visitor visas - General & Family. Do read through the information to understand what type they should apply for.
http://www.ukba.homeoffice.gov.uk/visas ... /visiting/
http://www.ukba.homeoffice.gov.uk/sitec ... sitors.pdf
Getting a family visitor visa is no more difficult than getting a general visitor visa. Additionally depending on how old your niece is, you may also want to read the information published for Accompanied child visitors:
http://www.ukba.homeoffice.gov.uk/visas ... uirements/
The two key requirements are:
1) That they can maintain and accomodate themselves during their visit, without recourse to public funds. This can be done either by using their finances & or your finances if you intend to sponsor their visit.
2) That they intend to return to their country of residence at the end of their visit. This can be done by showing strong ties to their home country. Some examples of how this can be done are: Employment, Education, Family ties, Property and any other commitments they have to where they live - I.e. anything which will show their intent to return...