As what i heard from the seminar held last sat by Ming Pao....the daughter of your sister and her husband can entitle to the mark of "first kid" as their is a NEW family.
I believe as long as there is a official marriage certificate for the couples..it doesnt matter who will be the applicant as it is again a NEW family. In fact, i think should use whover that can gain more marks from the school applying as the applicant.