、。。
But that's perfectly natural. After all, you don't yet know their manners.
Literal
But, that [topic-は] extremely obvious thing is. Their [genitive-の] manners [object-を] you [topic-は] still don't-know [explanatory-の][because-ですから].
A reassuring explanatory remark. 至極 (しごく) is a register-marked intensifier meaning 'extremely,' 'utterly' — much more formal than とても or 非常に, and usually reaching for a sententious or reassuring tone. 至極当然 ('extremely natural,' 'only to be expected') is a common collocation. In the second sentence, notice the topic stacking: 彼らのマナーを is the fronted object, and あなたは (the listener) is then introduced as the topic-subject. Japanese is freer with topic fronting than English. The ending ~のですから combines the explanatory の with the reason-marking から, giving 'after all, [it's the case that] you...'