Insult

"Your parents were brothers!"

Kash:
indeyamati (iyalesa) mambrela! (the verb is optional)
/inde-ama-ti  i-ale-sa  mambre-la/
 mother-father-you (3pl-be-past) brother-pl

(There would be debate as to whether indeyama-- singular in form, plural in
meaning-- takes a sing. or pl. verb.  Colloq. speech could get around this
by omitting the person-marker; officialese avoids it by using a quantifier
or number-- yuno indeyama 'all parents', indeyama loroni 'both parents'
would require a pl. verb; pipinal indeyama 'each/every parent' would take a
sing. form-- or the otherwise unused plural form indeyamala, or even by
breaking the compound form into individual words-- pipinal inde (i) ama.  In
fact, indeyama in most cases probably best translates as "parental couple",
since it could cover m/f, f/f and m/m. A f/f couple could indeed have a
child by artificial insemination, but the guys are just out of luck, however
fervently they might wish otherwise.)

So it's technically translatable, but wouldn't be much of an insult, at
least not in the way it is in Engl. (where you are the product not only of
an incestuous act, but an unnatural one at that).  The Kash would be upset
only by the incest part.

A far worse insult (fightin' words!!) would be
amati (or, indeti)  feliyoç!!
your-Fa (your-Mo) (is/was) a Gwr

Sexual contact with the other species is beyond taboo, it is inconceivable
and utterly repugnant, probably like  telling an Arab that one of pers*
parents was a pig or dog.

*Yes, j'adore "Woman on the Edge of Time"!