In order to create a genogram, Mosaic needs specific information about how people are related. You can add a step-parent to step-child relationship, but this can't be shown in the genogram from that information. The genogram needs to know which parent to attach the step-parent to as it will draw this as a partner (or ex partner) of a parent.
In order for a step-parent to appear on a genogram correctly, they should be added as a partner (any relationship type configured as partner:partner type in reference data) of the relevant parent. If the parent and the step-parent are no longer in a relationship, the relationship can be given an end date. This will show the relationship as ended in the genogram but will still show the step-parent.
Mosaic is configured with a standard set of partner:partner relationship types, your system administrator will be able to make amendments and additions if these are required.