I'm no lawyer, but it seems that if you're getting the vacation time that was offered to you when you accepted the job, it doesn't matter what other people get.
At most jobs, people recieve a written "offer" if they are offered the job. The amount of vacation time is part of this offer. If the employer wants the employee bad enough, they may offer more vacation time, more pay, ect...
Sueing an employer because family members get more vacation time than others, is like sueing because everyone isn't being paid the same.
I hope there is more to this potential law suit that just unequal vacation time. If this person feels they deserve more vacation time, a lawsuit isn't the way to get it, just ask.