“I don’t know the relative well enough to know if they would have told my GF that she needed to tell me ( if it is true ) first and my GF just didn’t.”

This is a real possibility, too. That jumbled thought process from the relative makes it seem more likely the info about your GF’s addiction is true, to be honest.

You seem to believe the relative.

So – it sounds like in fact you do believe there is some credibility to this relative’s claims. Or that it’s at least in the realm of possibility.

You are long overdue for a discussion with your girlfriend here. I think to an extent, her reaction will be telling (although – at the same time I’m sure she’s going to be pissed and deny it – which really isn’t that telling because if someone was going around saying I was a heroin addict I’d be pissed and deny it too – because it is ludicrous).

You need to talk. And put a pause on any live moves until you feel confident.

The bottom line here seems to be that, for whatever reason, you don’t trust your gf. You can’t buy a house together under those circumstances. You also didn’t trust your gf or her ability to have a reasonable discussion enough to tell her what the relative actually said. You have agreed with a relative, one she doesn’t get along with well, to keep secret and internalize/raise doubts about your gf and your relationship what would be a hugely nasty, scurrilous lie if it in fact isn’t true. You have no way of knowing who is being truthful, but it seems that you largely believe the relative. That says a ton about the state of your relationship and lack of trust in your gf.

It sounds like your gf is a trans-woman, which might be the actual cause of this relative’s animus.

It would have been vastly better and easier to tell your gf the whole conversation right after you got off the phone. Now it will be awkward and your gf will have reason to distrust you and to doubt your loyalty. And you’ve also sentenced yourself to days of helplessly stewing about it.

This really could be a relationship killer, which may be exactly what this relative intended.

My GF is a born that way girl. I should have said what the relative said-the full story-but I was shocked and in disbelief. It felt like if I said it straight out,from this relative, it would too hurtful-I don’t know,I guess if true, and likely there is truth there, to lesser or more degree, than I am very upset that my GF never told me the whole truth. and I am still mad as hell at the “helpful” relative.