Adding a Sibling
To add a Full Sibling to the Selected Individual click on the Add Button on the Navigation screen toolbar and select the a Sibling menu item.
Once you have entered the known details of the Sibling into the Add Person Display, Click on the Finish Button at the top right of the screen.
You must have previously added or connected parents to a Selected Individual before you can add a Sibling.
Half Siblings: The above method is applicable only when adding full Siblings. If you want to add a Half-Sibling then you should make the shared parent the Selected Individual and add the Half-Sibling as a child of the shared parent and follow the prompts to connect the other parent.