My sister complains of her nose and breath smelling constantly, what could this be from? The foul smells in her nose bug her more than anything

Sinusitis. Sounds like sinusitis to me. How long has she had these symptoms? Does she have history of nose allergy? Any condition that blocks off the sinus drainage channels, including colds, allergies, non-allergic rhinitis, deviated nasal septum and nasal polyps, can cause sinusitis. You typically get thick, yellow, foul-smelling discharge, nasal obstruction and pressure around eyes, nose and forehead.
Nose & breath odor. I would suggest pt to see an ent. Bad odor may suggest infection or bleeding. There could be an infection in the sinuses or the nose associated or not to foreign bodies, to necrosis, to abscess, to staphylococcus or to deep fungal infections or even cancer.
Nose/breath smell. How old is your sister? Foul smells in the nose of children are sometimes associated with a foreign body (bean/peanut/toy) in the nasal passage. Sinus conditions/infections can also be associated with foul nasal discharge & bad breath. Have her see her fp or an ENT doc to check for these possibilities. Tooth decay with dental abscess should also be considered.
Cacosmia. Bad smell: could be sinus proplem, teeth/ gingivitis or chronic bronchitis/ empysema, more important to check brain as some tumour in frontal lobe part can cause abnormal smell.
Usually the sinuses. Bad smells sensed in the nose are usually from a sinus infection. A doctor can look inside the nose to see what is up there, and then prescribe antibiotics as needed.