Can sinus/ear infection cause partial loss of vision?

Yes: This would be seen in an acute complication of a sinus infection - a peri-orbital abscess which is surgical emergency. If this happens, you must be evaluated for a possible abscess.

Unlikely: Ear infection cannot lead to visual loss. Sinus infection rarely can escape the sinus and enter adjacent tissue including the nerves of vision. Basically for this to happen you would have to profoundly neglect your sinus infection for a lengthy time to allow such penetration. You should see your otolaryngologist if you suspect your sinuses or ears are infected.