Is my marriage to a Western girl, with whom I had an illegitimate relationship and who then embraced Islam and became upright, valid despite my family's opposition to the marriage?

If both of you have repented to Allah with sincere repentance from the illicit relationship, then keep your wife. You are not obligated to divorce her to appease your parents. Strive to please your parents and explain to them that your wife has adhered to her Islam, and that what she did was out of ignorance and misguidance, and that she has repented and embraced Islam, and you fear her reverting. The Messenger of Allah (peace be upon him) said: "Indeed, Islam effaces what came before it." Continue to seek their رضا and supplicate to Allah to incline their hearts towards your wife.