Health insurance laws vary by state. Individual companies, insurance brokers, and corporate policy may dictate how these laws are interpreted.
The first portion of the question addresses divorced spouses. As a general rule, no, the divorced spouses are not eligible under the existing policy. This may vary with individual companies; however, for the most part no. Once a divorce is finalized, the laws of health insurance are very clear- the ex-spouse cannot stay on the same insurance policy. You may qualify for temporary insurance under the COBRA Continuation Health Coverage program. For more information on the COBRA program, visit The Department of Labor.
The second portion of the question addresses step children. Dependents, under the age of 25, are protected under the Affordable Care Act. Step-children do qualify under this category with a few conditions. The dependent must be the biological or legally adopted child of the policy holder. The law also allows that these step-children may marry and have children of their own while remaining on the health insurance policy. The parent policy does not cover the dependent's spouse or children. There are extensions to the age limits which vary by state.