Animal Crossing New Horizons Mabel Sisters are the clothes sellers in the game. You'll meet Mabel when you first go into Nook's Cranny, where she will tell you she will stop by periodically to sell clothes.
Animal Crossing New Horizons Mabel Sisters: How to Unlock the Clothes Store
After you unlock Nook's Cranny, Mabel will appear randomly like other visitors to your island and will set up a stall in your town square in front of Resident Services. You can buy a small selection of clothes from her.
After she visits three times, Mabel will tell you she wants to set up a permanent shop on your island. You'll get the kit to decide where it will go, and then the day after construction starts you'll have the Able Sisters clothing shop.
Since Mabel's visits are random, you might get the Abel Sisters sooner rather than later. If you're unlucky (like me), you might not even see her for an entire week.
Once you get the clothes shop, Animal Crossing veterans know to talk to Mabel's sister, Sable, every day to get her to open up. It's now known what reward you'll get, Sable needs a friend anyway.