Well there isn't a thread about PC families. It's just something people like to do in there signature when people on PC want to be brothers, sisters, cousins, etc. However, there is a pairing thread to where you post up your very close friend in which you guys want to be paired. If you do want to be paired and you guys agree, one of you guys post it up there. It's in the Other Chat area I believe.
Anyways to answer your real question, no there is not a thread for them. People would rather just post their PC families in their bio, signature, etc. Hope that helps.