So the past two times I've had my sim marry the Effenmont heir, they've decided their home is in the woods now and won't go live in the castle. The heir will also only come to the castle if invited, and will leave if left alone for five seconds. I feel bad trying to divorce them though so idk what to do. Is there a fix? A mod or some kind of workaround I can use?

TIA

No, no fix. I am not unhappy about this though, as I have played TSMP&N for simply years, and have quickly grown to heartily dislike the Effenmont princess. She is unfriendly, and completely cold no matter what traits she has in any particular game I've played. I nearly resent the game writer's for making a forced marriage quest in order to achieve certain goals, or ambitions in the game. The male twin may likely be the same as the female, but only after the quest to annex Effenmont is over. Then the REAL personality of the Effenmont twin you married comes out. Perhaps you may be glad that they spend all their time in the forest once you realize the awful people that they are and how they will treat your monarch, once you see it for yourself.
Or you might want to do as I do every time my wonderful monarch has to marry the Effenmont princess....Before the actual marriage takes place, I take control of another hero sim other than the knight or the spy, whichever method you play, I've played both, and the one using the spy to help the monarch is kinder to that sim in that they do not require the spy to duel the king (who is usually in my game much higher in level), so your spy does not suffer in the way that your knight will in negative buffs, as well as sim pain, which you may already know.
I usually take control of my Peteran Priest right before my monarch and the female twin get to the church to marry, then I keep control of the priest, and let the quest come to an end. Immediately you will be back in game play mode, not in kingdom mode, and you will still be in control of the sim you took control of during the quest. For me, the Peteran priest. Now, what you need to do is take back control of your monarch. Have her talk to the twin she married, select MEAN, then at the top of the options is the choice to annul the marriage. Click it. Other npc sims may show like they are thinking about your monarch in a negative display, but worry not, there is no effect in the game from the thought bubbles above their heads. Although if any of your hero sims are present during this break-up, each of them may get a red buff for seeing it, and I think its negative 40, but you can remove this bad buff from them by taking control of them, pressing left click on your mouse, and at the same time press shift on your keyboard while pointing at choice of sim, (later do the same to release them.)
To remove the buff after you have control of a sim hold your mouse pointer over the buff you want removed, and left click it, while at the same time press the Ctrl button on your keyboard, and release.
Monarch is no longer married to a devil, and can pursue a possible loving marriage with a sim they choose with whomever may bring them happiness. For I promise you an Effenmont twin never will. So think of your monarch first, before a sim who only will bring suffering and pain to your sim, and possibly you, because isn't your monarch really a part of you? They are your first sim you created for your kingdom. Certainly you have fallen in "love" with your sims? They need you to care for them, as they make no decisions without your input.
You will still have control of Effenmont after the quest and after annulling the marriage if you choose to try my method, so worry not. Release all sims in control after and you will go to kingdom mode. When you start a new quest you will still have control of the sim from earlier, so if they are not a participant of the new quest, remember to take control of them during responsibility time, and do their respos so they do not get a shirk.

Good luck to you, and may your sims always find the happiness and joy they deserve.

Bless, Xeny