Is saying "I must do such and such" considered a vow, and is my saying to my sister "I don't have to wear tight hijabs" considered a vow, or a mandatory vow that is not binding?

The statement "I must give such and such in charity" or "I must not wear tight clothes" is not considered a vow (nadhr) because it lacks the specific formulation of a vow. Even if it were to include the formulation of a vow, a vow is only valid for something recommended (mandūb). Wearing tight clothing is originally forbidden (harām), and refraining from it is an obligation (wājib) in itself. Therefore, a vow to abstain from it is not valid.