Putting aside the fact the LW must not be very close to her brother, either physically or emotionally, for him not to know she's been with a woman for 2 years, why isn't her problem easily solved by coming out to him but asking he keep it a secret from Mom and Dad? Clearly they aren't going to discuss it with him, so how'd they know if she did tell him?

And how exactly did she get caught basically agreeing to Mom and Dad's demands? Are they loaded so she's fearful of being cut out of the will? Because otherwise I don't see what's standing in her way. Their relationship is strained at best and they've made it clear that SO isn't ever going to be welcome in their home.