Hello, Pauline!
Click the "Edit" tab on William's profile and look for "Edit Family" on the right-hand side.
Click the "edit father" link; on the new page, locate and click the "remove" link or scroll down to the bottom of the page. Click the long green button that says "REMOVE ROBERT AS FATHER OF WILLIAM BEECROFT" (I may have accidentally clicked the button; if so, you can skip to the next step!).
Repeat the process for the mother: click "edit mother" link, scroll to bottom of page; click green button "REMOVE MRS ROBERT AS MOTHER OF WILLIAM BEECROFT."
Removing both parents should also disconnect the sibling relationships.
Let me know if you have trouble - or if I detached the correct father!!