Why would this girl say to me , Well you are not my aunts daughter?

My uncle got remarried last year. His new wife my new aunt lol she is amazing we become close. Now there is issues she has her 27 year old niece helping her clean the house cooking and watching her kids for her during quarantine because my new aunt sometimes she has to get things done in her office. Any way I wanted to stop by give her a cake for her 44th birthday yesterday. Surprised her niece gives me this vibe that I am unwanted there. I went into my new aunts kitchen to make myself some coffee and her niece has this serious face ask me what am I doing and why am I graving things without asking my answer to was she is my new aunt I am part of her family I can make myself at home. She looked very upset as if I was being intrusive this is her responds. Well you are NOT! her daughter you now and walks away. Why would she act like that with me as if I am invading her property? The problem is her niece is always there present living there just to take care my new aunts house duties. My new aunt told me not to take it personal that she is her favorite niece that she can joke around. I don't want to ruin my friendship with my new aunt. What should I do?
Why would this girl say to me , Well you are not my aunts daughter?
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