I'm currently doing an essay on the history of Hawaii and I was wondering, regarding international law, the "legality of Hawai'i".

Well, the USA already officially acknowledged that the overthrow was illegal...

In 1993, the US Congress passed a joint Apology Resolution regarding the overthrow; it was signed by President Bill Clinton.

The resolution apologized and said that the overthrow was illegal in the following phrase: "The Congress—on the occasion of the 100th anniversary of the illegal overthrow of the Kingdom of Hawaiʻi on January 17, 1893, acknowledges the historical significance of this event which resulted in the suppression of the inherent sovereignty of the Native Hawaiian people."

The Apology Resolution also "acknowledges that the overthrow of the Kingdom of Hawaiʻi occurred with the active participation of agents and citizens of the United States and further acknowledges that the Native Hawaiian people never directly relinquished to the United States their claims to their inherent sovereignty as a people over their national lands, either through the Kingdom of Hawaiʻi or through a plebiscite or referendum".