1)
Does "Ish" refer to one man?
1.
Radak: It was not only one man, like "v'Ish Ba mi'Ba'al Shalishah" (Melachim II, 4:42). Rather, it is any man, like "Ish mi'Zar'acha", "v'Ish Ki Yiten Mum ba'Amiso" (Vayikra 21:17, 24:19). The same applies to "v'Ish Es Kalaso? v'Ish Es Achoso."
2)
Why does it say "Inu" regarding his sister?