Your mother's cousin is not your aunt or uncle. In the traditional sense of family relationships, aunts and uncles are siblings of one's parents. Since your mother's cousin is not a sibling of your mother, they do not fall into the category of aunt or uncle.
The correct term to refer to your mother's cousin is "first cousin once removed." This means that there is one generation difference between you and your mother's cousin. Your parent and your mother's cousin are first cousins, and you are one generation removed from that cousin relationship, making you first cousins once removed.
To summarize:
- Your mother's cousin is your first cousin once removed.
- Your mother's siblings are your aunts and uncles.