Looking into the Affordable Care Act rules, an individual cannot receive the tax credit if they are "able to get affordable coverage through an eligible employer-sponsored plan." Here, affordable is defined as "the portion of the annual premium you must pay for self-only coverage...does not exceed 9.5 percent of your household income." Regarding spouse/dependent insurance, it says "The affordability test applies only to the portion of the annual premiums for self-only coverage and does not include any additional cost for family coverage." The rules for employers state that "Employers...must either offer minimum essential coverage that is affordable...to their full-time employees (and their dependents)" or pay a the IRS. (It also notes that for the purposes of this rule, spouses are not dependents.)
Thus, as far as I can tell, (certain) employers are required to offer affordable health insurance to (certain) employees and their dependents, where "affordable" is measured only by how much is charged to the employee for their own coverage. In particular, there is no standard of affordability for dependents or spouses of the employee, and the employer can charge as much as they like for this insurance and still be considered to offer "affordable" coverage to the employee and dependents.
This idea seems to be confirmed since (as I just tested) submitting to the marketplace at healthcare.gov that one family member is offered free health insurance through an employer leads it to ask whether the other family members are also eligible to enroll but does not ask how much is charged to them should they enroll. The web site will make a determination that the whole family is ineligible for the tax credit based on this information.
Something seems broken here--either my understanding of the law, the law itself, or the marketplace. If there is a mistake in the above reasoning, could someone please point it out? If there is no mistake, could someone please explain the intent of the law here (to the extent it is known)?