أحسن الله إليكم If a sister has her menses fasts to make up from Ramadan, does she have to make those up before fasting the voluntary of dhul hijjah? And if she fasts the day of Arafah with the intention of making up the obligatory fast, will she still be rewarded for fasting on Arafah?

Yes, she has to make them up first, and her intention has to be for making up obligatory fasts. As for if Allah rewards her more for the day of Arafah, then the bounty of Allah is expansive and Allah knows best. But her intention must be for making up the obligatory fasts first, and she starts with the obligatory fast that she has to make, not something that is voluntary.