In my opinion, he needs to leave. He's tried to get ok with you dating his brother, but he can't. He's asked you to stop dating his brother, but you don't want to, and it's not his right to force you. So, if he doesn't like the situation he's in, he needs to leave it.
We don't get to control the behaviours of others. We only get to control our own behaviours. He has 4 choices for his own behaviour:
- get ok with it
- stay without liking it, but stop making a fuss about it
- stay without liking it, and keep making a fuss about it
Making you leave Mark is not on his list of options. So far he's been choosing 4. It is his perogative to keep doing that until he drives you away, but it won't be pretty. While it's true that leaving Mark is on your list of options, that's not at all up to Nevermore and he has to stop thinking it is.
You seem firm that you don't want to choose. So it's up to you to show him that you're resolute, that you are NOT going to make that choice, and that you want him to stop asking for it IMMEDIATELY because it's disrespectful and hurtful and manipulative. In other words, stop letting him believe that making you choose is actually on his list of potential behaviours.
Every time you come here and try to find any other way out, you're sending him the message that he just has to try harder, find the right angle, push just your right buttons, and eventually you'll give in. Let him know that it ain't gonna happen and if he really feels that the situation he's in has got to end, then it's his responsibility to end it by walking away. Period.