Can a stepson steal a will and make any changes with out the legitimate wife knowing?

The only person who can legally change a will is the testator, i.e. the person whose property is being disposed.  Neither the testator's wife, stepson, nor anyone else can do so.  If, however, the testator decides to change his/her will, he/she is not obligated to inform anybody else (unless he has a contract to do so, which is rare).