How do you become friends with Sable at the Able Sisters' shop?
Don't give up: keep trying to strike up a conversation with Sable each time you visit the shop. Every day you speak to her, she'll begin telling you more about herself and her sisters, engaging in lengthier conversations and divulging more details as time goes on.
She'll also become friendlier towards you, and want to begin showing her appreciation for your attentiveness - in the form of hand-sewn patterns for use in furniture customisation.