You should immediately file a first information report (FIR) for the offence of extortion and criminal intimidation against your wife and her family members.
Don’t wait till the filing of their case against your family members. They have committed an offence of extortion which is punishable under Section 384 & 506 of the Indian Penal Code. Hence, you should lodge the FIR immediately.
So far as child’s custody is concerned, you should file a case under the Guardian and Wards Act. This suit shall be filed in the city where your child is living currently.