If you don't know Japanese relative clauses yet, please learn them first. If you already know relative clauses, 仲間でも家族でもない is just another relative clause that modifies それ (="it"). ～でも～でもない is simply "to be neither ～ nor ～".

If it's really the very first sentence of the story, no one can understand what それ refers to at this point. Probably それ will be explained later in the story.

仲間でも家族でもないそれは魅力的に映った、とても。
It, being neither "buddy" nor "family", appeared attractive, really.