A man marrying his brother's maternal half-sister who is his father's stepdaughter
If the reality is as you mentioned in regard to the relationship between you and your wife, your marriage with her is in accordance with the Shari‘ah (Islamic law).
Your wife being your maternal cousin, your brother’s maternal half-sister, and your step-mother’s daughter (Part No. 18; Page No. 364) does not constitute a relationship that prohibits your marriage to her by authority of Shari‘ah. There is no Nas (Islamic text from the Qur’an or the Sunnah) that declares it to be Haram (prohibited); the basic ruling is for permissibility.
If the reality is as you mentioned, that you made the oath of divorcing her twice (irrevocably) or thrice (irrevocably), to do this or not to do that, without fulfilling your oaths - in fact you broke them - and, as you did not really intend to divorce your wife, but meant to encourage or prohibit yourself from doing something, you should offer Kaffarah (expiation) for each oath you did not fulfill. The Kaffarah is to feed ten needy people or clothe them, or free a slave. It will be sufficient for you to give five Sa‘s (1 Sa‘ = 2.172 kg) of wheat, rice, corn, or whatever you feed your family on, to ten needy people; each of them taking half a Sa‘ for each time you did not fulfill your oath. If you cannot do this, you should perform Sawm (fast) for three days for each unfulfilled oath. If you do not know how many oaths you made, try to estimate them, and offer Kaffarah for the most likely number of times. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.