All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.

 

If the case is as described in the above question, then it isn`t permissible for the questioner to marry (X) because she was suckled by his grandfather`s wife, making her his grandfather`s daughter in suckling and his paternal aunt in suckling. The evidence on this is that  Narrated Ibn 'Abbas (RA): "The Prophet (Peace be upon him) was offered to marry the daughter of Hamzah. He said, "She is unlawful to me for she is the daughter of my brother in suckling; and what is unlawful by reason of blood relationship is unlawful by reason of suckling relationship." [Agreed upon]. And Allah the Almighty knows best.