I am assuming you would set the privacy as red. It would be the safest choice without having their permission.
It would show her/them as "private wife", "private son", "private daughter", etc. It won't obstruct anything already there. It may require that the profile be set to green privacy, but I would need to take a closer look at the guidelines to be sure.
It would show that the information has been researched, perhaps saving time for those down the road. As well as that relationships do exist, more information than is currently represented.