I'm offended by the remarks my sister made to me tonight?

A little backstory-

My sister gave birth to her daughter a year ago and I see them about once every two weeks (always been that way since she was born) and my other sister/her husband were named her god-parents.

Tonight when all of us were heading up to visit my Dad and I offered to bring up one of my nieces's books and that's when my sister snapped at me by saying "I feel like you are trying to step in and control my daughter! Also that your tryingher to prove yourself"

Her comments perplexed me because I would never try to step in and take her parents role in her life, I never try to tell my sister how to parent her daughter. I never step in and do anything without my sisters permission and I always ask her first, for example " would you like me to give her a strawberry?". However, yet its ok for my other sister to step in and give her baths, take her for walks, offer to watch her for the night, give her food etc... and nothing is said to her yet she thinks' I'm overbearing and acts like I'm some kind of threat to my neice.

Am I trying to prove things? Sure I guess, I mean I do try my best to be a good uncle to her, but that's about it.

What should I do about this?