High risk is usually given if the results indicate a risk of greater than 1 in 300 that the child has the condition. In Australia, people are usually given the actual figures (e.g. 1 in 241), and your sister should be able to request this number.

Approximately 5% of women receive an increased risk result following screening.

The rating is a combination of the mothers age, the measurement of the fluid at the back of baby's neck and measurements of chemicals in Mum's blood.

Chromosomal variations (such as Down syndrome) occur more often in older mothers. Babies with chromosomal variations can have increased fluid at the back of their neck (this can also be seen in other babies). The chemicals they look at have been found in higher/lower concentrations in Mum's who are carrying babies with chromosomal variations. It is the combination of all these things that gives the rating.

The high risk number (1 in 300) is only used because they balance it against the risk of losing a baby following confirmatory testing (CVS or amnio). The stated risks of losing a baby following CVS is 1 in 100 (1%) and following amnio is 1 in 200 (0.5%)

You can get more info on screening here
VCGS Pathology

and there is a brochure at the bottom of the page that has a pretty good explanation.