Are you the child's primary caregiver? If not, when you say "what do we do?" do you mean how should you react? Join in! She knows it is just a doll. She is just getting a kick out of playing with a life-sized avatar.
yup...normal ...has the other twin any doll friends ?
This is not uncommon - and she knows this doll is a doll. Apparently, this "pretense" is a sign of a creative and bright child (or so I've read). In a few months, perhaps when school returns - she will forget about this "pretend" friend. I once referred to our granddaughter's friend as "imaginary" and she replied "no, she's real, just invisible". She knew and so did I. All the best ...