What are the reasons why a woman and/or family would want to know the gender of a fetus in a twin pregnancy? When during gestation can the SNP-based test be performed?

What are the reasons why a woman and/or family would want to know the gender of a fetus in a twin pregnancy? When during gestation can the SNP-based test be performed?

What are the medical, social, and familial reasons for why a woman and/or family of would want to know the gender of a fetus in a twin pregnancy? At what time point during gestation can the SNP-based test be performed?