Property purchased by father registered in mother name can she sell it

Sir my mother is committed with my cousin and she is ready to sell her property for him.. But that property purchased by my father in the name of my mother.. Problem is that my mother she has the registry of that property of her name.. That property was purchased in 2002 and we have a blank 100 rs. Stamp with her signature of year 2004.. I was minor at that time but now i am adult. Is there any way to stop her by using that stamp.???