wow well from what's been going on and posts lately... I don't doubt anything.
I would just clarify that for me, "overshadowing" is a very positive thing, it's like a big bro/sis with an angel wing and they take you under it and show you the ways of the Christed self.... so I wouldn't mix the term with possession. But I'm not saying I got the book on it or anything, I haven't a clue as to whether someone else defines overshadowing in a broader way that can be good or not-so-good.
And what you describe sounds like possession or even demonic influence to me, especially if it's different from her usual self, and how different she is to your kids. If these are really a new side to your gf, that appears when sis is around, then looks to me like a lower-vibe spirit, earthly or demonic, is strongly associated with lil sis and probably been with her a long time. With the family tie it knows just how to influence your gf because it probably knows her well, too. Sounds painful to watch, I'm saddened you are going thru this. My heart goes out to everyone involved.
I would advise you get some prayer support. I know it sounds so frightening but all the power lies in the lack of awareness and hence ability to protect against them. They are not strong spirits; they are weak and just knowing this, visualizing sending them away, asking your angels and guides for assistance and protection - it's usually very effective.

Of course if your gf believed and would it admit it that's what would help the most. If only she could recognize how she was being influenced and choose to ignore it/send it away and seek other influences from the spirit realm, that's what would probably be most effective.