Maybe one of them could be sick or dying?

Maybe one was treated like crap by crappy adoptive parent(s)? Or one of the adopted parents died and it was the kid's fault so the living step-parent hated them for it and could never forgive her?

Maybe one twin has emotional problems and is always flying off the handle - causing all kinds of problems (usually she winds up causing her own problems because of this and as a consequence it always seems to interfere with finding out information or having someone help her, or it caused a document to be destroyed)?

Maybe records were destroyed by fire, or tornado, or some other natural event.

Maybe one, or both, have a deadly peanut allergy? And somehow they both wind up eating the same dish that happens to be tainted by peanuts without their knowledge (either by mistake or was it done on purpose? If on purpose, who would have reason to want to off one or both sisters?)

I could get more specific detailed ideas, but I don't know your story better. But just go wild - first try to picture a simple description for your story (just dumb it down - just for this exercise), clear your mind, maybe stop thinking of it for a while, then think of the simple description of your story (kind of looking at an image through a camera lens fuzzy and out of focus) then just go crazy and think of the wildest things you can think of to really shake up the story. Think of things to sabotage your story, or help get it back on the rails. Do the craziest ideas - things that would never ever work - and the more you do this - something of a real idea that would work will pop up in your mind - something you could work with - maybe less crazy, but who knows?