You need to use some of the less well-known features of Diagrams to display multiple parents.
By default Diagrams only display the first set of parents regardless of whether they are biological or not.
Usually, the first set of parents are the biological parents but in your case they are not.
One option is to change the order of the Spouse tabs so that whichever parents you want in the Diagram are shown first.
After saving the Diagram as a PDF you can swap those Spouse tabs back by using Edit > Undo.
Another option is in the Diagram to use Diagram > Insert Into Diagram and choose a suitable tree to insert additional parents.
e.g. Select yourself as an Ancestor Tree root and FH will ask which parents to display.
After tidying the layout, save the Diagram as an FH Chart to preserve the multiple trees, otherwise you must start from scratch each time.
If you initially choose yourself as the root of the initial Diagram then FH will ask which parents you want to display.
Similar options exist for your siblings.