Your mother's first cousin would be your first cousin once removed.

你母亲的表亲一旦被移走，就会成为你的表亲。

So, my first cousins, many of them have children; they are my first cousins once removed.

所以，我的堂兄弟， 他们中的许多人都有孩子；他们是我的堂兄弟姐妹。