what can i do if my dad is dieying and his wife (stepmother) inherits all his things what can the children do about this

If you r dad has a will - then his will governs what happens to his assets.  If he does not - his estate is considered "intestate" (without a will) and then the second wife only gets a portion of the estate, not the whole thing.  You should find out if your dad has a will.  If he does not, and he is competent, talk to him about doing a will.