I suppose it depends on what you mean by coerce. If by that you mean that the parent was not independantly legally advised, was perhaps not competent to sign any legal document, and entered into this transaction despite its being improvident, then there is a possibility that the transfer could be set aside. Although it would mean an immediate court action and costly fees if the sibling contested the matter.
However if by coerce you mean that the parent, of their own free will, having had independant legal advice and being of sound mind, voluntarily signed over their property to their child 'behind the back' of another child, then of course the parent is entitled to do whatever they want with their property, whether that be to sign it away to one child, or the cats home or whatever.