Steps to reproduce:

• open/create Qt Quick UI / Application
• switch to Designer
• create a hierarchy of some items
• select one of the inner items and hit 'Become first sibling of parent'
  • this item becomes the last sibling of parent
• select one item that has siblings (but is the first on the hierarchy level) and hit 'Become child of first sibling'
  • nothing happens (I'd expect this to become child of the next possible sibling as it can't become child of itself)