When a spouse dies and everything is left to wife or husband, can step kids contest

Heirs can contest a Will, the question is whether they will succeed.   Stepchildren are not heirs, so if you are talking about the estate of a stepparent, then the stepkids have no rights of inheritance.  But if you are talking about children disputing their own parent's Will that leaves everything to a spouse,  if the children are able to prove that their parent was manipulated into signing the Will or the Will is not valid for some reason, then they may be successful.  It is a difficult thing to prove, however.