I can find no way to alter relationships , such as step parent instead of parent , or half sibling . I have 10 half siblings , my mom had 1 out of wed lock , then married my dad had me divorced married 2nd man had 8 more ,my dad remarried had 1 son in that marriage ,I would like for all relations to be accurate in their amount.
My personal opinion is to just put each child under that child's respective parents and let the chart speak for itself, except for adopted children who I would put under their adoptive parents also.
The other option is to put all step children under their biological parent and step-parent as well as both biological parents.
In Family Tree relationships are defined as a child's relationship to parents. So two people cannot really be marked as half-silbings, that would just be inferred by having a parent with more than one marriage and children by two different spouses.
You set relationships by first adding a child then clicking the pencil icon next to the child's name in the Family Members section which opens the parent-child relationship pop up:
This opens another pop up where you add or edit the relationship type:
This allows you to create complex family structures: