Upon your wife's untimely and intestate death, all her properties, whether movable or immovable shall devolve equally on her own class I legal heirs.
In your case you and your son are the class I legal heirs to succeed her estates as successors.
Your in laws cannot hold the properties or the title documents of the immovable properties and they cannot refuse to hand them over to you.
You are the natural guardian to your minor son, hence you can e the custodian of your child as well his share in all types of properties that belonged to your deceased wife.
If your parents in law or brothers in law refuse to hand over the properties that belonged to your wife, you may issue a legal notice and then file a suit for recovery of possession of the same.
You may even lodge a criminal complaint against them for refusing to give your properties to you without any valid reason.
Your father in law cannot cancel the registered gift deed executed in favor of your wife during her lifetime, now after her death, it will be illegal an invalid i the eyes of law.