Ok...so, Dh and I have been together ten years.  SD13 has always been a quiet kid whom I like a lot.  She's generally thoughtful and kind.  The only thing that's ever bothered me about her is that she's not entirely truthful;  she will lie about totally unecessary things to anyone she thinks will be displeased by whatever it is. It takes a lot of careful and sensitive handling for her to come out of her shell, and she'll retreat very quickly if you slip up. 

 We have full custody.  I'm the person she historically tells her troubles to; she sits close to me and follows me around. 

The problem is that she's suddenly trying to get between me and DH.   For example--DH and I have always sat on the loveseat together when he gets home from work.  We'll put our feet up and chat.  We've been doing this for years.  Then one day, a month or so ago,  I came into the room preparing to settle in with DH like usual, but SD13 was already there in my spot.   She's crumpled up against his side, her face hidden in his armpit,  her legs folded up and leaning into him in a full-body press, and I think something must have upset her so I leave to give them some privacy because she appears to be confiding in him.  Nope, turns out she just popped into my seat immediately when he sat down, there was no reason or anything wrong.  He thought it strange too, but being a loving dad he just put his arm around her.

Then she began this full-court press of blocking me like this every single day.   I've been edged out on movie nights, for meals, and every day when he gets home from work.  Now they sit together on the love seat with their feet up. If DH gets up to go somewhere, she'll jump up and follow him very closely, into the kitchen or out the door.  If he's cooking she'll move in to help.  if he's gaming she wants to game.   She moves very quickly, like she's guarding a resource. She will even sit on the loveseat before DH is home and wait.  In response, he and I began sitting together across from her on the other couch, but she just moved her tactics to that spot.  
 

If I do happen to get to sit by him--I don't try very hard, because I hate the weird feeling that she's competing, and I don't wish to engage--plus I'll admit I was hoping it was just some weird phase that would pass and that no reaction from me was likely the best course of action---she'll  sit close by and stare, like she's watching us on tv. And not  like she's jealous of me, but like she wants to BE me. I know it sounds weird and it kind of freaks me out, to be honest.  If I get up to get something, like some water, she'll jump in and steal my spot.  She doesn't even try to hide it.   
 

DH is evidently feeling weird about it too, because now whenever I come into the room at random times throughout the day they are sitting on the couch like that, looking like a couple, pressed together, and he looks up guiltily, like I caught him doing something. He literally can't sit down anymore without her instantly pressing up against him.  I emphatically don't believe anything weird is going on--DH is just poor with boundaries and telling her no, and he's afraid to hurt her feelings.  She's one of those super-sensitive kids that needs a lot of accomodations at home, certain foods only and needing things around her to be a certain way, and she does remote school due to social anxiety.  She's suffered from suicidal ideation in the past and we've had her in counseling and on prozac for over a year, but she still refuses to sleep in her own room (she sleeps on the couch right next to our bedroom door).  If I'm having an off day, I can be in the kitchen doing dishes and she'll instantly assume I'm mad at her when I'm not even thinking about it, but she'll still go tell her counselor that I am.   It's oppressive, to be honest, having to maintain a "customer service" face all the time at home, and not getting anywhere in our genuine attempts to make her more comfortable.
 

Today, she took my seat in the front of the car.  Just like that.  That was a first lol.  I'd talked to DH about it and told him I was going to start setting boundaries if he wasn't, so I kindly kicked her out in a joking manner.  She immediately capitulated, but then went off across the parking lot by herself anyway the rest of the time, and was silent the rest of the day. 

As an aside, this isn't a case of SD not getting any time alone with DH; she gets a lot, more than DD7.  But now she wants all of it, and I can't figure out why so suddenly.  She certainly wouldn't admit to it, and will likely completely retreat if we try to talk to her about it.   I've known her since she was three, and know her well. She's never acted even remotely like this.

I just wondered if anyone else has experienced a miniwife like this one, where a good SD-SM relationship suddenly got weird overnight.  She wears my clothes too, and repeats whole phrases that I've said like she's assimilating my thinking and opinions wholesale as her own.  I don't get a malicious vibe from her at all,  just a benign "Single White Female" kind of thing.  Like she's identifying with me to the point that she wants to step into my skin and assume my role.

DH has said he'll do something about it, but I sincerely doubt it.  He's just too sweet and soft.  And I don't want him to think less of me for complaining about SD's behavior, but it's affecting my sense of belonging in my own marriage.  And, I never thought SD herself would "turn" on me, but it feels like she is, by pitting her herself against me.  I have always been her biggest advocate and supporter. 

Has anyone had this sort of thing happen and then resolve?  SD13's adult sister was a contemptuous miniwife from hell, who was definitely very hateful toward me, but SD13 is not and never has been.  Am I doomed?