Am I selfish for not buying a medicine for my sick sister?

My sister is sick and asked me to buy her tylenol or advil I refused to buy her and told her I will not go to drugstore. Tbh I went to drugstore and bought my ointment but didn't buy her medicines she needed. I went home and she saw me holding an ointment I bought from drugstore so Am I selfish?
Am I selfish for not buying a medicine for my sick sister?
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