Sudden eye irritation, itchy, blur, strained feeling, sneezing, sinus irritation, headache, runny and stuffed nose... my sister thinks it's allergy?

Allergy or cold: could be either, or both, but colds are more of an acute nature, and can describe the symptoms mix that you have, at any rate, medical treatment is the same. Rest, fluids, pain killers, antihistamines, unless contraindicated, saline nasal washes, if no improvement see your doctor, if recurrent or prolonged, see an allergist/Immunologist, best wishes