Yeah she is.
Is the stoyry different for each town? SOmeone said the abandoned her and someone said she was adopted. Mine said they were poor and their parents died, Sable took charge then Labelle and Sabel had a big fight and she left to go to the city.
To unlock it youh ave to:
Get Sabel you friend
Buy at least ONE thing from Tom Nooks
Buy one thing from the Able Sisters
Get three check ups from Gracie
Talk to Labelle once
Then I think you can unlock it.