Spouses have a lot of rights where it comes to the Law, and possessions.  If your Spouse were to die, you would automatically be intitled to any and all financial assets.. retirement/  social security benifits, life insurance, 401k, plus anything that you aquired together during your marriage.  The only way I could see the In-laws legally taking anything in the event that one of you dies (without a Will), is if something were to have been promised personally.. and then it would also need to be in writing, and notarized. 

Never the less, it is always wise to have a Will, especially if your already feeling concerned about the In-laws. That way you will both be assured that nothing will become ugly, or time consuming!