Well, first Da Twins wants loyalty but at the same time do damage to the one who is responsible for the demise of their Father and the last War ending in their loses, making harder their lives, so, a way they do is trying to get them separated of each other (Alexia and Rowan break up). X'zaralt is like another passive force in contrast to Da Twins, because she's there not because Da Twins but because Rowan's there.... So, initially her interest is Rowan, she admire and idealize him, seeing him in a sad state isn't an option for her, so helping the couple acting like an ointment to heal a wound, a guide to maintain each other together even if corruption becomes necessary is better than seeing them suffer, of course, there are ulterior motives because she could in theory try to visit Rowan during the time of peace, try to seduce him disguised as a commoner woman, there's story about why X'zaralt finally decided to move to Bloodmeen to see Rowan when she could had a lot of chances to do it before, it seems meanwhile she's trying to help the couple, she moves more close to Rowan in a more romantic way (even unexpected to her, because if she wanted to do it wouldn't be necessary to approuch them as a couple). In relation to why main characters npc's can't take by force over them,
Rein and his team (Venus Noire) have been wisely avoid sanctions from Patreon, as they stated:
But, they are on a thin line as the overall story in the background does, even if the whole team comes here to shout us that isn't, there's commenters that signaled that in the story (and some of them very funny as you can see

):