Step-brothers are non-Mahram

You haven’t mentioned any details in regards to your family in your query, hence, making it a bit difficult for us to comment. However, we assume that your mother married a person that had a son from a previous marriage. This son is now married and has a family. In this instance, he will be considered as a non-Mahram to you, and the both of you will be required to observe Purdah from each other. Similarly, you will be required to observe Purdah from his male children that have reached the age of Buloogh (maturity).