Without known ancestry she's considered a Domestic Longhair (DLH). She does appear to have some features similar to the Siberian breed but that doesn't necessarily mean she has any Siberian in her. Siberians were developed from naturally occurring cats, so you can expect to see some lookalikes in the random cat population. Whatever mix she is, she's gorgeous.
Her name is "Catty LaBelle". And yes she is beautiful. I hope to learn a lot about her so that I can be a good Mommy to her. That's why I was asking what breed she might be... I'm sure her former mommy knew but got so sick that she had to be surrendered and no one knows for sure now. She has a VERY thick blunt coat and a magnificent tail and the prettiest scruff under her chin. But most of all she seems to be a sweetie.
She resembled greatly (in real life -- in cat person, I mean) a Norwegian Forest Cat... in size and appearance...but as you said, without knowing her ancestry we call her a domestic long hair.