i think this might be a translation term unique to this manhua (cmiiw), but they explained it in a translator's note in earlier chapters. it's like "senior" or "junior," a way of referring to the people ranked below or above you in the cultivator world (irrespective of age, so that's why some Penglai cultivators who are older than Qu Chen still refer to him as "martial uncle").What even is "martial niece"?