| I don't think it's up to you to say anything. It's your GF's responsibility, because it's her family. I have to say, it is rather weird. I wonder if they would do this if she was with another man?
I suspect that some people might see a lesbian relationship as not quite real, or just something that happens while you're waiting for a man to come along. Your GF's family is probably in this boat and view her relationship with you as a strange moment of madness.
Your GF has to be really firm say to her family - "marshmallow is my partner now and I would like you to invite her and I to any functions. I don't mind if you invite my ex, that's your prerogative, but marshmallow and I are together and I would appreciate it if you invited her and I as couple. Ex and I are no longer together and I don't want to be included on an invitation with him". Perhaps your GF can also tell her family that she won't attend functions unless the invites are extended to you as well.
I feel for you, it's horrible feeling like the third wheel. |