If someone is only a half-sister to the bride, the proper term to address the groom would be "half-brother-in-law." This term acknowledges the fact that the person is related by blood to the bride but only partially related to the groom.
It's essential to use the correct terminology to avoid any confusion and to accurately describe the family relationships. However, in more informal settings, some people may simply refer to the groom as their "brother-in-law" without specifying the half-sister connection. The level of formality in the language used can vary depending on the context and the relationship between the individuals involved.