I'm sorry to hear about the hypothetical situation you've mentioned. The term "brother-in-law" typically refers to the husband of one's sibling, such as a sister's husband. If your sister were to pass away, her husband would no longer be considered your brother-in-law, as the marriage tie is broken.
If he were to remarry, his new wife would become his new sister-in-law, but not to you. However, some families might maintain close relationships even after the death of a family member, and if that's the case, you may still consider him as part of your extended family and refer to him as a friend or "former brother-in-law" out of respect and affection for the time he was married to your sister.
Ultimately, how you refer to and perceive him after such an event depends on the dynamics and bonds within your family. People may have various ways of addressing these situations, and it's important to be considerate of everyone's feelings and preferences.