Your mum's cousin's brother is your second cousin.
In genealogical terms, a second cousin is someone who shares a great-grandparent with you. Here's how the relationship breaks down:
- Your mum's cousin is your first cousin once removed. This is because your mum's cousin is one generation above you (your mum's cousin's parent is your grandparent).
- The brother of your mum's cousin is also one generation above you. He is the child of your mum's aunt or uncle (depending on whether your mum's cousin is the child of her parents' sister or brother).
- Since you and the brother of your mum's cousin share great-grandparents (your mum's grandparents), you are second cousins.
So, you and your mum's cousin's brother are second cousins.