Most business owners and top managers who allow this to happen have no idea how much damage it causes in the fabric of the enterprise. When good management practices are not adhered to, and when conscientious employees feel that rules that directly impact them are not being followed (e.g., work schedules), an atmosphere is created in which productivity, quality and profitability suffer. That is, there is no genuine impetus for otherwise industrious employees to work hard to excel when/if the cards are already stacked against them being treated fairly and rewarded properly.
I always recommend a specific policy prohibiting people who are related or dating to report to one another. But it is probably a bit late to put that practice into place where you work. You neglect to say what your position is in the company; perhaps it would put your own job at risk to follow this advice. But what needs to be done is for you to put this same question to "stepmom." That is, how does she feel about her stepson's lackadaisical approach to his job and giving preferential scheduling treatment to his current girlfriend?
She may be too busy to really notice her stepson's behavior and/or the fact that he is skirting established work scheduling guidelines because of his personal involvement. She may even be prompted to tell him to find a job on his own--and take his girlfriend with him.