、。
My youngest brother may not be in the wrong, but neither am I.
Literal
Youngest-brother [topic-は] not-bad maybe, but I [also-も] not-bad.
末弟 is a formal kinship term for the youngest brother (literally 'end brother'). 悪くない idiomatically can mean either 'not bad (in quality)' or 'not at fault / not in the wrong,' and the disputational context here makes the latter reading clear. けれど is the softer, slightly more formal/feminine counterpart of けど (and stands below the more formal けれども). The parallel structure with ~も emphasizes balance between the two parties.