Aqua, I agree w/ Ginny that DS' and DIL's hiding the truth about who your sister is from GD is "bizarre behavior." It's also poor judgment on their part, IMO. I realize it's a difficult topic to handle, but I think they could have found a better way. Nothing you can do about that, of course.
She'll figure it out eventually, so IDK what they think they're "hiding."
Maybe if you follow their rules w/o argument or complaint, they will come to trust you more, in time, and loosen up the reins.
I'm sorry none of your family members have come to your defense. But they probably don't know the full extent of the problem. That might be just as well, though, b/c their getting in the middle would not likely do anything except make matters worse.
Dolcecatte, you said: "... so far as I can make out, (ED) doesn't see anyone else, as she is not working, has lost touch with her friends as well as family, and lives hundreds of miles away."
The "has lost touch with her friends as well as family" sends up a red flag for me. Apparently, she has moved far away, and that may be why she has lost touch w/ people, etc. However, it's generally easy to keep in touch these days via FB or Instagram or whatever. Are you sure she isn't doing this? If not, sorry to say, I'd be concerned. (I wouldn't say anything about it though at this time.) Unless she has pushed others away b/c they've tried to intervene on your and DH's behalf, as frequently happens nowadays. Then I would understand why she "lost touch" even if I didn't agree. Regardless, now, I'm more glad than ever that you and DH are meeting w/ her. Best of luck in advance!