Here's a new one from Captain Caption.https://captaincaption.deviantart.com/art/Twin-TG-700186705>>7576704
It also happens to hit a lot of the high points for twinning myself.
Reality changes so everyone believes they've always been twins.
The original wanted a twin, or is at least open to having one.
New one grows to like their new form and new sister, and becomes similar in mind, either slowly or immediately.
And while it isn't necessary, having some slight differences between them is also nice, as is having those changes reference the original form in some way. But still having them be similar enough and close enough to pass off as one another.
I know not everyone likes reality changes or identity death but if it's done right it's fine.