The World Trade Organization rules
The World Trade Organization was established on January 1st, 1995 by the Uruguay Round negotiations. On July 2016, the WTO included 164 countries.
The WTO is altogether an organization for trade opening, a forum for governments to negotiate trade agreements, a place to settle disputes and it also operates a system of trade rules.
Those rules are trading without discrimination, lowering trade barriers, promoting stability and predictability, promoting fair competition and encouraging development and economic reform.
Qatar, Saudi Arabia, the United Arab Emirates and Bahrain are all members of the WTO and must therefore respect the WTO trading rules between each other.
State sovereignty is a fundamental principle of international law. Under the Convention on International Civil Aviation (also known as the Chicago Convention, 1944), each State has complete and exclusive sovereignty over the airspace above its territory. Hence, article 28 of the Convention specifies that States are not obliged to provide air navigation services over their territory themselves. Yet, when and where States elect to provide facilities and services to support international air navigation, these facilities and services must comply with ICAO Standards and Recommended Practices.
Because most families of the Gulf countries have ties in each GCC country, the air ban imposed by Saudi Arabia, the U.A.E and Bahrain against Qatar causes painful situations to those families. Human rights organizations such as Amnesty International have already voiced their concerns.