OK, did some testing and I think I figured out where the bug lies. During the second day in the city when you officially become Hand of the Queen, your actions regarding Aunt/Sister has an effect on what options are available to you before the feast in the evening. If you can skip directly to the feast, Sandy MB route correctly triggers. But if you do not and do another action (e.g. visit Brothel) before hand, it will apparently always trigger the Sandy romance route. And for the Aunt MB route, you don't get an option to skip to the feast (only visit Aunt and visit Brothel), so the Sandy MB scene at the feast will never correctly trigger there.