In my text book it is given that when 2 pith ball is connected with nylon thread to a charged rod they repel each other

My question is that nylon thread conducts charge

@ aniket 
Yes , nylons are thermoplastics and condensation polymers , 
if u could refer to the internal structure of nylon , then u wil know that it have free hydroxyl end by which it can conduct electricity and transfer heat from one place to anathor , 
for that reason through that nylon wire , the charge flows to the two pith balls , and acc to the property of electromagnetic indcution the same and like charges developes in each pith balls , and they starts repel each other .. 
HOPE IT CLEARS YOUR DOUBT 
ALL THE BEST ..