Fiscal policy can be divided into two 'sub-policies' - spending and taxation.
Spending
The more a government spends in social welfare, the more equitable income distribution will be (obvious). General increases in spending, i.e. expansionary fiscal policy, tends to reduce unemployment. A reduction in unemployment largely impacts lower income earners, because they tend to be the section of the labour force prone to cyclical unemployment, while higher income earners are less affected.. As such, expansionary spending to reduce unemployment tends to improve income distribution.
Taxation
Progressive taxation systems (taxing higher income earners at a higher rate), adopted by most of the world, improves income and wealth distribution significantly. The more progressive a country's adopted taxation system, the lower the GINI co-efficient (0 gini co-efficient is perfectly equal income distribution). Conversely to before, contractionary fiscal policy, such as lifting tax rates, disproportionately impacts lower income earners, because rising unemployment blah blah blah.
Fiscal policy has a very dramatic impact on income distribution. Above is only some of the impacts.