Under IRC 414(b) and (c), businesses under common control are treated as a single employer for retirement plan rules. The two main patterns are a parent that owns at least 80% of a subsidiary, and a brother-sister group, where five or fewer people own at least 80% of each business and more than 50% counting only ownership that is identical in each.
Ownership is counted after family attribution. You are generally treated as owning your spouse's interest in a business, with a narrow exception when you have no ownership, job or management role in it, and the interests of your children under 21. Adult children and parents are counted only if you already control more than half.
This is the main way a solo plan stops being solo. If you own your consulting firm outright and also own 80% or more of a business with staff, those staff generally count as your employees, and a plan that covers only you would fail the coverage rules. A separate rule for affiliated service groups (IRC 414(m)) can link service businesses with little or no common ownership, for example a professional corporation that is a partner in a group practice.
The outcome turns on exact ownership, so it is a question for your tax adviser before you start a plan. See who can have a solo cash balance plan.