The Grand Duke was the Reigning ruler of the Duchy of Hesse-Darmstadt, until it was abolished after WWI.
Only his wife or widowed mother held the rank of Grand Duchess. Wife - Grand Duchess. Mother - Dowager Grand Duchess.
The Grand Duke's oldest son and Heir (only males can inherit directly, very similar to the Pauline Laws in Russia)'s wife was titled “Hereditary Grand Duchess" like a Crown Princess will held.
Elisabeth was only a daughter of a Grand Duke, so she was in-titled Princess. Even though she was an only child at her death, she was not her father's heir.
Unless AND A BIG WHAT IF, she lived, only child, and married the next inline heir to the Duchy of Hesse-Darmstadt were the Princes of Hesse-Cassel, a junior branch of the Hesse Family..anyways if Elisabeth married the eldest boy and heir to both branches, THEN she would have been a Grand Duchess.
A bit late, Mandie, but thanks for the explanation.