If you dream your brother-in-law has new wife, it might mean your sister would be without a husband. However, you should not decipher dreams literally as they appear in dream imagery. In dream symbols, a new wife can mean a new hobby because it takes up time, money, energy and other resources. Just like a new wife would. So if you dream your relative has a new wife/ husband, the dream interpretation can mean they have new interests.
Dream interpretation depends on the personal background of dreamer, culture, situation in life and associations with the imagery seen in the dream. If you dream of other people, you should do your own checks to verify the claims in your dream. We can not interpret dream symbols literally as they appear.