All the pieces are falling into place. Maggie requested a financial audit, it could be related to divorce proceedings and division of assets. That audit may have triggered a deep dive into finances, potentially exposing money that was embezzled. That would be a problem for Alex. Furthermore, several properties were in Maggie's name, meaning she had control of selling those properties in anticipation of division of family assets.
Paul was tangled up in a very problematic lawsuit (boating death) that was going to cost the family millions.
Elimination of Maggie might put Maggie's assets (property) in Alex's hands, depending on the Will. Elimination of Paul eliminates legal costs that would have been needed for Paul's defence.
Alex had enough time to drive from the hospital to Moselle, back to his mother's house to check on her, and back to Moselle to discover the bodies. Alex could have requested that Maggie and Paul stay at Moselle to be closer to the hospital to have a final visit with his father. Alex discovered the bodies shortly after they were shot, but stated that he was not concerned about a shooter in the vicinity.