Myself, I remember being raised specifically to not pursue specific hobbies in order to give my brother a space to grow outside my shadow.
Now clearly you know both brothers and you clearly understand their sibling dynamic, yes? That they can both love you and each other without any problems, right? Or is there any unresolved jealousy that needs to be addressed? Because you've mentioned Mark is afraid of being kicked out, which while unfortunate is a real threat? I'm not saying Neverwhere is in the right, just that he already has an existing relationship with Mark that is stronger than yours with either of them.
That aside, about the incest: Mark and Neverwhere cannot have sex simultaneously with you without there being an incestuous atmosphere. Perhaps you've never considered that as a possibility? I know I'm not interested in my brother's sex life.