If my mother's Will states that she leaves my stepfather her house and upon his death it goes to me and my 2 brothers, can he sell or alter the house?

This depends on how your mother's will is written.  Unfortunately, you have not provided sufficient detail to frame an accurate answer.

If your mother's will leaves a "life estate" to your stepfather, or deeds the house to your stepfather "for life," then he inherits a right to live in the property until he dies.  He cannot sell the house without you and your siblings' consent.  He can "alter" the house, but he cannot "waste" it.  "Waste" is a technical term that means, in general, doing something that diminishes the value of the property.

If the will is not clear, then it will be up to the Probate Court to interpret it.  From your question, it seems likely that your stepfather will inherit a life estate, but it is possible that he could inherit the property outright.  If he inherits it outright, he can sell it.

With a life estate comes the obligation to pay the taxes, insurance, and maintenance on the house.  You and your siblings are not obligated to pay this so long as your stepfather lives in the house.  If he no longer lives in the house, you should consult a lawyer about transferring the house to you and your siblings.