The Dark Sovereign series follows the Del Rossa mafia family and currently has 11 books: three trilogies and one duet.

Here’s the important part:
Each brother’s story is either a trilogy or a duet. Each trilogy or duet can be read as a standalone story.

You can start with Alexius' trilogy, Nicoli's trilogy, Caelian's duet, or Isaia's trilogy. Whichever one you want. You don’t need to read any other brother’s books first.

However, within each brother’s trilogy or duet, the books must be read in order.

But, for the best reader expeprience I do recommend you start with Alexius.