Why did my hamster bite me?

My older sister decided to pet our hamster today and we have had it for two weeks now and we have never touched or held it. I have put my hand in his cage and he has sniffed me and never done anything to me. My sister put some hamster food in her hand and stuck her hand in his cage to see if he would hop on but instead he bit her! He did not draw any blood but could he have felt threatened? I know they are partially blind so could he have mistaken my sister's hand for food? She also lifted his little igloo to get him out. Also will this bite affect my sister's health? She has already washed her hands with soap and water.