Suppose you are preparing to marry your soul mate, only to belatedly realize that this person is not truly yours alone? Beyond telling them apart, there is a unique set of issues that go with marrying someone who is the product of a multiple birth. It demands a special sensitivity to family dynamics and, in most cases, a tolerance for the special closeness among the siblings. People marrying into this situation must recognize that they may never have their partner fully to themselves. Forget about it. Spouses may turn first to their siblings in times of emergencies or to share deep secrets. Hadassah Holmes, 29, a lawyer in the Bronx, is to marry Ben Holtz on March 15, but she is also very close with her twin sister, Yaffa. Holmes said. Multiple births have been on the rise.