I thought of that. My girlfriend is Ultramarine on this site, although she rarely plays. I would mention in my Profile that Ultramarine is my girlfriend. It would still be fairer to people who might want to play Go with her. If they suspected that I was Ultramarine, then they could choose not to play her.
My issue is with people on Waiting Games that look like new players, but are really highly skilled, with multiple identities. There are also certain players that have thousands of games, then also play under different names too. If I knew a little more about the players that I am about to engage, I would refrain from playing them.
Also, it would be interesting for everyone to see the connections between players, even if they are family members.
I don't think that family members should play online for ratings anyway.