A brother-in-law is not excluded solely by that relationship. However, the worker must also meet the qualifying-employee definition: lawful work permission alone does not establish that, and nonimmigrant workers are excluded from this definition. Check the actual employment status and role as well as hours, without assuming that every relative on payroll creates a qualifying job.
Two different family questions in the same file
Keep the employment question apart from the immigration one. Whoever immigrates with you as a derivative — a spouse, or an unmarried child under twenty-one — is decided by age and marital status under the rules applying to the case, and each person is assessed individually for admissibility. A relative outside that group who works at the enterprise is a payroll question, not a derivative one, and the two records are prepared separately.
Conditional residence gives the whole family the rights of permanent residents for two years: the spouse may work anywhere, children may study, and no separate authorization is needed. USCIS petition fees from the G-1055 schedule, immigrant-visa or adjustment fees for each family member, the later I-829 fee, professional fees for source-of-funds and business-plan work, a targeted-employment-area analysis if relied on, and translations.