No. The issue is not living together (or not)--the issue is whether the proper legal relationship exists, which entitles someone to insurance coverage. "Girlfriend" is not a legal relationship--it does not convey any legal rights or obligation. If the children were his, he could cover them (even if not their mother, if he was not married to her), but if they are her children by another man, your brother has no legal relationship to them. (That is not to denigrate any emotional or financial relationship--he could be a father figure and he might help support them; but even if that's the case, there is no legal relationship.) Your brother would have to marry his girlfriend to cover her; he *might* also have to adopt her children, though he might be able to cover them so long as he marries their mother. He can contact the insurer and his company's HR department to discuss that.