Hi all. I'm new to this community....

This seems to be complicated to me.

I have a younger Vietnamese niece who lives in my home. I have just started to try and
learn vietnamese. Sometimes we text message each other in Vietnamese through the iPhone.

Sometimes I address her as "Chau gai" and sometimes as "Em". She said I cannot address her as "Em" because it means something else..."girlfriend"

I have heard men address younger girls as "Em" many times as well as girls address older men as "Anh" many times. They were not siblings and they were not in a boyfriend/girlfriend relationship. They were just simple friends.

My niece says because I'm her uncle that I cannot address her as "Em". Is that correct?

Here are my thoughts...
If I was her older brother it would be ok. If I was her older friend it would be ok. I can address other younger, unrelated female friend as "Em" (notice I said "female friend" not "girlfriend"). Am I correct on these usages of "Em"

I'm lost here. Can someone shed more light on the proper usage of "Em" and how I can better address my niece instead of "Chau gai"?

Without a direct word for "you" in vietnamese, I find addressing someone a bit difficult. After all, I don't want to address someones girlfriend as "Em" one day and end up with a blackeye.

I greatly appreciate any thoughts on this.