De er hjemme hos oss.
Is hjemme hos oss a set phrase？Can I say De er hos hjemme vårt？
And how to distinguish the word is Masculine or Feminine in order to choose the possessive form?
"hos" is used here to mean "with" and is used for "oss" or "us" http://www.norskengelskordbok.com/en/dictionary-norwegian-english/hos
"at home" = "hjemme"
"hjemme hos oss" is an expression that is used frequently in Norwegian and it is the name of a comedy too.
https://translate.google.com/#auto/no/They%20are%20at%20home%20with%20us.%0AThey%20are%20at%20our%20home%0AThe%20home%20is%20ours.%0A%0AOur%20home%0AOur%20home%20is%20here%0AThey%20are%20at%20our%20home. It is unusual for Google Translate to not translate word for word and I think that means a native speaker may have contributed the correct phrase.
I hope someone comes along who is a native speaker.
http://www.engelsknorskordbok.com/en/dictionary-english-norwegian/home "hjem" is neuter and not feminine nor masculine. You can find out the gender through Wiktionary which uses n for neuter, m for masculine and f for feminine: https://en.wiktionary.org/wiki/hjem#Norwegian_Bokm.C3.A5l