literal translation: your sister's pussy! this sentence in English language is not complete because the adjectival is missing. Howerver, in Arabic it is a complete swear word which means: you are a brother of a whore sister. The closest equivelant in English is Mother fucker kos ommak.
In Arabic culture it is very offensive to mention someones mother in a rude way, let alone his sister. The female members of ones family represent his honour and dignity.
Kos ukhtak manyookeh : your sister's pussy is fucked.
kos ukhtak sharmootah: fuck your bitch sister.