If the
women don't sit for iddat after Talaq is it lazim on a husband to pay money
for her iddat?

Answer: 

If a woman
marries someone while she is in Iddah either after divorce or after death, the
nikah is null and void and is unaccepted, according to all four madhabs . The
man and woman are regarded as living in sin. But what if they have two children
from this marriage. Is it watiy bishubhah or not. Will children be
legitimate.