Yes, Labelle is an Able Sister and her side of the shop counts as part of the Able Sisters' shop in the game.
As you become more friendly with Sable, she will reveal Labelle's backstory. This picks up where the story left off in Animal Crossing: City Folk, where Labelle was present in the game but not on speaking terms with Sable. Labelle's real name is 'Label' and is a sibling of Mabel and Sable. She left after their parents died, studied in the city with Gracie, and has returned in Animal Crossing: New Leaf to sell accessories in the family shop.
Buying everything in the accessory shop will accelerate the construction of Kicks in your town, as there is a higher quantity of purchasable items in that side of the shop. The two exterior doors aren't meant to distinguish the two rooms as separate shops so much as they are to decrease travel time for players not wanting to visit both rooms of the shop.