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.
彼は私にとって家族同然の存在です。
He is like family to me.
長年一緒に暮らしてきたペットは、もう家族同然だ。
The pet I've lived with for years is practically family now.
Compound of 家族 (family) and 同然 (just like, equivalent to). The expression is transparent and follows standard word formation.