The Medic
Sienna is Emma's sister, and Stella's partner. She was the first not-cat that Nestor had approached, being told of moving from the city to the woods.
Sienna's gift is medicinal; no matter what herbs she's given, she can always make something to cure almost any illness - including even the common cold. If she can't make something useful, she can at the very least say what she'd need to make something of use.
Her curse, however, stings of irony - regardless of how good anything she makes, none of it works on herself, and she is perpetually sick. None of her illness has ever been deadly, but it ranges in severity from day to day. It can make things far more difficult for her depending on how bad it is, and it also makes it frustrating knowing that nothing she makes will ever be good enough to cure herself.