Recently we had an audit of the system and you had to prove your domestic relationship. That came in the form of tax records, bill summaries, marriage certificates or something that showed you are your partner were somehow sharing a bill/home etc.
Good or bad the airlines policy has been very liberal in the past but they have chosen to reign it in a little.
As for someone with no parents in-laws are still not allowed.
You do not have to prove a domestic relationship unless they are registered as a domestic partner. That gets you other bennies beyond travel privileges (ie medical, etc). You can designate anyone you want as your travel companion, with no documentation necessary. However, if you are married or have a domestic partner registered, they MUST be your travel companion, no one else. It's one or the other.