Will a major uti affect ur brain function?

No: An extremely severe urine infection - severe enough to need hospitalization - could temporarily alter your mental status. A more common "normal" UTI should not.

It can.: Elderly people can develop altered mental states from seemingly benign causes including medications, unfamiliar settings and infections. Often times, a simple UTI that manifests as urinary urgency/frequency/fevers in a younger individual may actually present only as confusion in an elderly person. The confusion almost always resolves as the infection is treated.