That’s really odd, especially for it to happen in her early 20s and for many years. Its hard to imagine how the dynamics of their relationship worked if they were unable to be public about their love. I’d have trouble believing it could be over. “Starting” a relationship so late in life with your biological brother and only sustaining that relationship for a few years makes me think there could’ve been a past of abuse or at least sibling attraction that was never proper dealt with. I can’t imagine how hard it must be and how confusing things are for you both right now. I definitely would suggest therapy, either as a couple or just individually, it probably would be beneficial.