My daughter acts like a Toddler

Yes... The title is right. My daughter still acts like a toddler, let's call her Tilly. It started when her little brother was born and she was 7, let's call him Scooter. (That's not their real names). She was just jealous of her little brother, and that we spent more time with him.

Tilly threw tantrums whenever me and her dad would ask to do anything, like the chores we gave her. She refused to clean her room, liking it messy. She refused to make up her bed. She would just say no, like a toddler would. She gave us the toddler treatment. Good thing we got her to behave when she was outside of the house.

Some time later, I caught Tilly watering her Scooter's diapers and sucking on his pacifiers. I thought giving her attention would just encourage her more. Her dad and I let Tilly use them both until she grew out of the phase. We thought it was just a phase.

These days at age 13, Tilly still wears and uses the diapers. She also has the pacifiers. She still uses a nightlight to go to sleep. She gets along with her brother Scooter now. She mimicks her brother's behavior, witch means we've got a toddler and a smarter toddler on our hands.
She's barley a teenager and still acts like a toddler.

My confession is that we were too relaxed with her as a kid and now we can get her too stop. What should we do now? Tilly is a mess. Other parent have it easier.