What happens if you drop Mark?
Will Neverwhere live happily ever after with you? Will you be devastated?
What happens if you drop Neverwhere? Will Mark happily raise the kids? Will you be devastated?
What happens if Neverwhere drops you?
You state you don't want to lose your husband and you love him, and you say the same thing about Mark.
It's easy for Mark to not make any ultimatums because he really doesn't have any leverage. I'm not sure it's honest to say he wouldn't make you choose, because you haven't placed him in that kind of situation.
That aside, it sounds like you want the situation where Neverwhere and Mark get along.
Well, then the only real solution is for Mark to make it a point to try to resolve his situation with Neverwhere. They are, after all, brothers. It's always in his capacity to work with Neverwhere to share; it's how brothers have to act their whole lives (until they get so tired of it that they move away, at least).
I mean, is that even on the table? You say Mark doesn't tell you about their conversations. I can't tell you why Neverwhere is hurting, he has to be the one to do it. I can't tell you why he and Mark can't get along; Mark can (or, if he isn't sure, he can figure it out).
I'm sorry I diverted the conversation about incest and taboo, it's really tertiary at this point. There's only two places you can look:
Maybe it really is the fact that he is uncomfortable. Medication does in fact change your perceptions and personality.