There is no single standard American English pronounciation; a number of variations are allowable; e.g., KUH-muhn, KOO-muhn, KYOO-muhn, KYOO-min, etc. Old English was pronounced KYE-men. The Latin is pronounced koo-MEE-newm (cuminum); French is KO-meen (comin); and FWIW, Spanish is ko-MEE-no (comino).
The word itself is not Spanish, or Latin for that matter. Modern Spanish derives from Latin, and off all the modern Latinate languages is the most like Latin. You may thank the Visigoths. Muchisimas gracias, Visigothos.
Returning to the term "cumin," however pronounced, it appears to have derived from an old middle eastern language -- ancient Hebrew perhaps, kah-MOAN (כמון), or perhaps an older, mesapotamian ur-language.
If your Mexican mom pronounced it KOO-meen, she had some nefarious mom-type reason having nothing to do with her native language, or she was from ancient Rome and never let on. Moms are not only very devious, they think it's funny.