Sir,
There was some matrimonial dispute between the my cousin and his wife and his wife has file the false police case against him u/s 498-A.Thereafter his father has filed the case against his younger brother and his old age father( and both of them are innocent). Kindly inform whether his younger brother can file the case for defamation against girl's father. If yes, when and what stage he can institute the same?

yes he can file a case for defamation , but first he had to get aquitted from the case . until the case in pending filing defamation will not suit.

for filing defamation the you have to prove that the allegations were false.  This is possible only if you  are acquitted taht too on merits.