My father-in-law died suddenly recently. I do not know whether he has partitioned his property or left a Will. My wife does not tell me the truth because our relationship is strained. As a son-in-law do I have any claim in the property ? I want to prevent my wife's brother & sisters from looting the property without giving anything to my wife.

You have no claim on your father in law's property.

You have no say. Only your wife can have the claim.

My wife is rather softhearted when it comes to her brother & sisters. How do I stop her from losing her rights ? Can her siblings divide it among themselves without telling my wife ?

 As a son in law you have no claim in the father in law's property. 

your relationship with your wife is strained but you still interested in the property  of your father in law.Great

After the death of the father, all the children becomes eligible for equal share of the property. None of the children can dispose off the property without the consent of the other without partitioning.

You need to speak to your wife in a soft voice and take her to your confidence and convince her without blaming her siblings. Otherwise, let it be as it is.