As a basic starting point, with regard to writing wills, the general rule is that you can leave your estate to whoever you want to. (X could, for example, leave his/her entire estate to Battersea Dogs Home. A,B, C & Y have no
automatic entitlement to any part of X's estate).
After the death of a testator, certain persons have the right to apply to a court for the terms of their will to be varied if they believe that the will did not make reasonable provision for them. However siblings can only make such an application if they were being maintained, in whole or in part, by the testator immediately prior to his/her death. Otherwise they can't change the terms of the will (unless, for example, they could show that the testator wasn't 'of sound mind', when he/she signed the will):
http://www.legislation.gov.uk/ukpga/1975/63
Once Y has received their share of the estate, it is theirs to do what they want with (including giving some of it to A, B and C if they so choose). Nothing written into X's will can prevent any redistribution by Y.
While it's perfectly possible for Y to agree to such a redistribution prior to the death of X, I'd advise against it as things could get messy if the siblings fall out. However if Y wants to go along that route then all that is needed is for him/her to write three separate pledges along the following lines:
"I, X, hereby pledge that should I receive part of the estate of Y under the terms of their will, I will give one quarter thereof to [insert A, B or C] within one calendar month of such receipt or, should such a time period not be practical, as soon as reasonably possible thereafter. I further pledge that should [insert A, B or C] die before I can make such a gift, I will distribute the said gift equally between the children of [insert A, B or C] within the same time period.
Signed: Y
Dated:
Witnessed by: [Signatures and printed names of witnesses D & E]"