The Fordhams (Mr. Stepbrother #4)
Mila Loveline

To board member Peter Grout, though, Camilla’s just a dollar sign—and a threat. To get to her, he’ll have to go through me. The problem is, he knows my two weaknesses and how to use them against me. It’s my company or my stepsister—and he’s forcing me to decide which one to keep…and which to leave in ruins.
The Fordhams is book four in the office romance serial, Mr. Stepbrother. It’s a short story (not a standalone) about a billionaire who is also his stepsister's boss. 14,000 words.