expression, noun which may take the genitive case particle 'no'
like family; as good as family
Describes a relationship so close that the person is practically considered a family member, even without blood or legal ties. Often used with ～の, ～だ, or ～のように.
He is like family to me.
The pet I've lived with for years is practically family now.
家族 means 'family' in a literal sense (blood or legal ties). 家族同然 emphasizes a closeness that is equivalent to family without the formal relationship.
親友 means 'close friend' but does not inherently imply the family-like bond that 家族同然 conveys.
Compound of 家族 (family) and 同然 (just like, equivalent to). The expression is transparent and follows standard word formation.