There are two ways to accommodate for a step child in your estate plan if you want to ensure they are part of your estate distribution.
1) Make Specific Distributions to the step children allocating either part of an asset or some money to them before the remainder of your estate is distributed.
2) You can have them received an equal portion of your estate by selecting a list of beneficiaries in the Estate Remainder distributions. You can also choose to give your children and step children specific proportions of your estate by selecting Proportional to a list of beneficiaries.
Note: it's important to not select choices that only go to you children if you want them included in distribution but they are not legally your child in the eyes of the law. They are not considered your descendants and will not receive distributions as you have intended.