A Fistful of Sky

This is *not* the protagonist... or anyone in the story.

There is a family of "witches" that receive magical talents at the onset of puberty. The middle child and protagonist, Gypsum LaZelle, is extremely late on receiving her gift, and at the age of twenty, believes that she will never have magic at all. When her gift finally does appear, it turns out that unlike the rest of her family, her gift is to cast curses instead of ordinary spells like everyone else. While this may not sound like such a bad thing, the main drawback of magical powers is that they have to be used fairly frequently or the witch in question will build up an excess of magic until they become so poisoned by it. At that point he or she sickens and dies. This was the fate of a relative of Gypsum's, who refused to hurt other people with her curses.

The novel, written by Nina Kiriki Hoffman, deals mostly with Gypsum's struggle with finding a non-destructive use of her cursing ability and with her relationship with her family.

Tropes used in A Fistful of Sky include:

Blessed with Suck: Gypsum gets awesome magic powers... but the power of cursing means that she has to hurt someone, corrupt something or turn another magic awry if she wants to prevent herself from becoming sick.
The Call Put Me on Hold: Of the late bloomer variety.
Covers Always Lie: The protagonist is extremely overweight, with short curly hair and the person on the cover is definitely not her. Nor is that a scene from anywhere in the book.
Cursed with Awesome: Even with the large amount of drawbacks of the cursing ability, it's still manageable with creative usage.
Fat Girl: Gypsum. She also has issues with getting into relationships to her because of her weight and self-image.
Fridge Brilliance: The odd tenants/belief system held by witches is never named, but bears a lot of similarities to the Eleven Satanic Rules of the Earth of LaVeyan Satanism.
Functional Magic
I Never Got Any Letters
My Beloved Smother: Gypsum's mother uses magic to try to control Gypsum and get her to do things like try to lose weight (unknowingly to the point where Gypsum becomes malnourished and dehydrated) or force her to talk when she's being sullen. This enrages Gypsum's father.
Papa Wolf: After Gypsum's mother uses a whole slew of magic on Gypsum to try to change her, Gypsum's father chews her out and makes her promise to never use magic on Gypsum again. After that he allows Gypsum to attend boarding school to avoid being at home.
Puberty Superpower: The onset of magic is usually around puberty, but it can happen much later for some.
Quirky Household: Gypsum's father is down to earth and takes the role of The Chick in their family dynamic. Gypsum's mother clearly wears the pants in the household, and can even be domineering at times, but her father tends to neutralize the worst of it. Since he's an ordinary person who married a witch, he's the only one in their family that doesn't have magic ability, but somehow he has enough authority to act as the voice of reason. Gypsum's siblings are all adults except for the very youngest, but only the oldest doesn't still live at home.
Superpower Lottery: Magic tends to manifest differently for each person, with most people becoming talented in a specific area of magic. Opal, the oldest, is best in her use of glamours and transformation magic, and uses this to be a great Hollywood makeup artist. Jasper has a talent for languages and music and is in a band. Flint's magic is chaotic and can be used to travel large distances, and Beryl keeps her magic secret. Gypsum, of course, has the power of cursing.
Theme Naming: Gypsum and all her siblings (Opal, Japser, Flint, Beryl) are named after minerals.