Is the questioner's marriage to a Muslim Filipino woman valid according to Sharia, given that the questioner knows she was married to a Christian man and has been separated from him for more than 12 years without a divorce?

Congratulate the woman on the blessing of Islam. She should be taught the matters of her religion and treated with kindness. If the marriage took place with the permission of her guardian (wali) and in the presence of witnesses, and she was not in her waiting period (iddah), then the marriage is valid. However, if you married her while she was in her iddah, you must separate from her until she completes her iddah from her first husband, and then observes an iddah from you, after which the marriage contract can be renewed.