The U.S. Supreme Court has long held that the federal government has the authority to annex territories by joint resolution, such as in the case DeLima v. Bidwell (1901), 182 U.S. 1. The DeLima case is about a dispute over tariffs imposed on sugar from Puerto Rico, but it does discuss the annexations of Texas and Hawaii, which were both done by joint resolution rather than treaty. Local scholars who dispute the validity of the annexation of Hawaii typically challenge it under international law rather than United States law.

To clarify what OP said about the Republic of Hawaii supporting annexation, the government of the Republic was a semi-permanent form of the Provisional Government of Hawaii that held power in the immediate aftermath of the fall of the monarchy in January 1893, and was thus mainly comprised of individuals supporting annexation. In contrast, at least 21,000 Native Hawaiians signed the Kūʻē petitions, which were sent to Washington to ask Congress to reject annexing Hawaii; this almost certainly helped convince enough Senators to oppose annexation that a vote to ratify a treaty would have failed.

Incidentally, there's an argument that Native Hawaiians benefited overall from the political structure of the Territory of Hawaii, and that the relatively recent calls for Native Hawaiian sovereignty (since the 1970s) are a result of economic and social stagnation relative to other ethnic groups in Hawaii. (See this column on Honolulu Civil Beat by former Honolulu Star-Bulletin reporter Ian Lind.)

Are you just interested in the Legality of the 1898 annexation? Because that's a question someone with a background in law should answer. I could write about the history around the annexation of Hawaii, but I take it as a hawaiian you already know all about that?