Is the money taken by the questioner from her deceased father's investment income without the knowledge of her mother—who receives it—considered her rightful due (her inheritance), or is it considered theft?

The money that the deceased invested during his lifetime becomes an inheritance for the heirs after his death. If the heirs agree to keep the investment, its returns are divided among them lawfully, just as the principal would be. The wife is not entitled to monopolize it without the rest of the heirs, and this must be explained to your mother. If she insists on preventing you, then it is permissible for you to take your share without her knowledge, as this is not theft but rather taking what is rightfully yours.