If you have assets then you will need to plan for their distribution after you die. Having a consultation with a family lawyer will ensure that your estate planning is done correctly so it is legal and no one can step in to challenge or change it. You have a right to leave your assets to whomever you want, but the law says you have a duty to your family to support and maintain them, so they should not be left out.
That said, you can still leave gifts to charities and people who are not related by blood, at your discretion.
Step children cannot challenge your Will, but other family members may. If you leave out some family members but not others, those who are not included can challenge the Will. Even if people are included, they can still challenge the Will if they think they should have been left more. These are the people who can challenge your Will.