Presumably the husband died and the family didn't want a foreign maegi hanging around the place.
To further clarify: "Half of Lannisport used to go to her for charms and potions. She was mother to a petty lord, a wealthy merchant upjumped by my grandsire. This lord's father had found her whilst trading in the east." That's the passage that gives the most clues.
The other was "The Westerlings were an old House, and proud, but Lady Sybell herself had been born a spicer, from a line of up-jumped merchants. Her grandmother had been some sort of half-mad witch woman from the east, he seemed to recall".
I noticed it because I was wondering why Jeyne Westerling was included here. We have her standing up for herself, showing a bit of spirit, and impressing Jaime enough that he told her mother that Jeyne was worth ten of her. Now, Rob is dead, and it seems like Jeyne has been force-fed moon tea, or something like that, in order to assure that Rob would have no son from her. It would seem like her part in all this is done, and yet GRRM dedicates this little section to further developing her character. So I'm wondering if she might not have some further role to play.
If Jeyne's great-grandmother was a Maegi, and one still alive when Cersei was a child, it's not inconceivable that Jeyne may have met her/learned from her. Or it might be that she has some latent talent passed through her blood line. Either way, with magic starting to blossom again in Westeros, I thought that her future role may involve some sort of magical ability, especially since these clues about Maggy and the Spicers were only added in this novel.
