The bolded part is what I'm unsure about. As a little sidenote, seeing that this is a manga and it's read from top to bottom, it may be of importance to note that 「生い立ちは騎士の家柄」 and 「ふたりとも戦で失い」 are on two seperate lines so to speak - whether they're supposed to be one sentence or not I'm not sure as there's no obvious space between them other than the "line break" due to missing space.
Here is how I interpret the sentence: "Brought up in a household of knights, they have both lost their parents and siblings in battle and ended up without any relatives."
If I were to reformulate the sentence as follows, would it retain the same meaning? 「生い立ちは騎士の家柄。ふたりとも戦で親と兄弟を失い天涯孤独の身となった。」 What I want to know is why 「ふたりとも戦で失い」 comes before the fact that their relatives died. I assume that 「親も兄弟もない」 is what's referred to here that they've lost and what is essentially being said in a different way is 「ふたりとも親も兄弟も戦で失い」.