Sounds like you're handling things well and perhaps it's a learning experience for you too. She's never been 11 before and you've never had to deal with an 11 year old niece (I'm guessing). Around that age children are becoming aware of their separateness from the others. It's not an easy time.
I can understand not wanting to undermine your brother's authority as the Father. It's actually good that you're not. It's his house and his family. That said, your niece has to know she can't treat you any way she likes. As you said, there are boundaries and she needs to learn that people have them.
I can understand not wanting to undermine your brother's authority as the Father. It's actually good that you're not. It's his house and his family. That said, your niece has to know she can't treat you any way she likes. As you said, there are boundaries and she needs to learn that people have them.