당신 sounds awkward in a social setting, and 측면 is a technical sounding word.
You can say it like these:
- 신랑 측에서 신랑의 쌍둥이 형제이신 X 씨를 무대로 모시겠습니다.
- 신랑 가족 중에서 신랑의 쌍둥이 형[동생] X 씨를 무대에 모셔 소개드리겠습니다.
* 측 = side (쪽 is another common word you can use).
* 쌍둥이 형/동생이신 = who is a twin older/younger brother. 쌍둥이 형제 is just "twin brother".
* X 씨 = Mr X.
* 무대로 모시겠습니다 = will invite (them) to the stage.
씨 is the most common title word for both men and women. For young women(like 15 to 25), 양 may also be used. For men or women over thirty, 님 would be a good choice - it shows more respect.
One thing to note is that the relation is more important in such a setting, so the name and title are come after the word for relation, as in 신부의 삼촌(이신) X 님을 모시겠습니다 (I will invite the bride's uncle Mr X). Sometimes the name might be omitted: 신부의 오빠 분을 모시겠습니다 (I will invite the bride's older brother). 분 can follow the term of relation for young people, and 님 if they're older.