Not sure if it's a bug or a lack of a feature, but I am unable to add siblings to my characters without both parents present. Some of my characters don't have parents or only have one. I want to add siblings regardless of parentage. Parentage shouldn't be a requirement
Create any character and link any existing second character as sibling, leaving the parent fields (biological and adoptive) empty.
Family Trees
This is how the system currently works and has worked in the past, therefor this is not a bug. This will be addressed in the next update of the feature in the future