Why is princess anne after prince Andrew and prince edward?

Presumably you mean why is Princess Anne after Princes Andrew and Edward in line of succession?

If so, it is because the current Law of Succession states that male issue of a reigning sovereign take precedence over female issue. Thus, Princess Anne, the Princess Royal, was second in line to the throne until 1960 when Prince Andrew was born, relegating her to third. Subsequently, the Queen's youngest child, Prince Edward relegated her to fourth place. Since then, both Princes Charles and Edward have male heirs, relegating Princess Anne still further.

However, in October 2011, at a meeting of the sixteen countries of which Queen Elizabeth II is sovereign, Prime Minister David Cameron proposed a change to the Royal Precedence Act, which received tacit endorsement from the other 15 states. This change, if ratified would instaure the principle of primogeniture, and Anne would take precedence over her younger brothers, although obviously Prince Charles, who is her elder by two years, and his son, William will remain heir and heir apparent. But, in order for this change to take effect, it must be ratified by the Parliaments of all 16 nations of which the Queen is sovereign. Wikipedia states it thus:

"The Commonwealth realms-the United Kingdom, Canada, Australia, New Zealand, Jamaica, Barbados, the Bahamas, Grenada, Papua New Guinea, the Solomon Islands, Tuvalu, Saint Lucia, Saint Vincent and the Grenadines, Belize, Antigua and Barbuda, and Saint Kitts and Nevis-are all equivalent in status, and independent of each other, while sharing one monarch in a constitutionally equal fashion. As such, changes to the succession laws, according to convention, must be identical in and approved by each country, though the procedure varies between realms."