'Whenever' here means every single time. 'Usually' means often but not always. So both words contradict themselves. A better sentence would be without the word 'usually'.
yes, whenever means every single time so I feel it would be best to say when I see a cat I usually start sneezing. They have not made it clear that they always sneeze.
Sounds like something an Eastern European trying to speak English would say tbh.
so your allergic to cats?
Perhaps it should be, "Whenever I am around a cat, I start sneezing." I am not sure why just seeing a cat would make her or him sneeze unless her allergies are psychosomatic. However, you could keep it as, "Whenever I see a cat, I start to sneeze. " in order to indicate a cause/effect relationship.
I'm ok with marionfox , it's better to tell this: when I see a cat I usually start sneezing,
Does seeing a dog or a cow make him yawping, I wonder?