There was something Alice and Helen both were unaware of, and that was the Teapot Dome scandal that erupted in '23. Once this scandal broke out, the stock prices of the major oil companies were bound to be severely impacted. Moreover, at that time, their three companies would have to cede control over three major oil fields. Then, Donnie could follow his original plan and purchase shares in the four major oil companies while using Sinclair Oil Corporation as a foundation to take control of one oil company.
By then, although Donnie's status in the oil sector wouldn't match Rockefeller's, he would at least possess the right to speak on equal terms.
Furthermore, by that time, the oil pipeline transportation system would have been completed, and even Rockefeller would not necessarily have an advantage in competing over this matter with Donnie!
Of course, it was a massive gamble.
If successful, Donnie would firmly establish his footing in the oil industry.