So. No wizardry. In that case, she'd just have to be valuable some other way, prove she was worth feeding even without magic.
When the next harvest comes, she's old enough to help. She does everything she can, but she can't swing a scythe like her brothers, and she's exhausted within half an hour. She goes to thresh the grain, and again she's clearly weaker than everyone around her, and quickly collapses. Her father beats her, and she gets back to it, but every day she's worse, the aches growing, her failure ever more apparent. She hears people saying it's the schooling, that if she'd been ignoring the books instead of reading them she wouldn't be so useless. She can't tell them they're wrong. She isn't sure they are.
The harvest is still enough to feed everyone that year, and the next, but it's getting worse. The next... still fine, but less so.
The year Silvia turns 13, the harvest is bad. It's not the worst it's ever been, but taxes are always higher than last year, even as the priest tells them to pray to Asmodeus. He doesn't seem to be helping, she carefully doesn't think. She eats last at their table, and every say she's a little more tired, a little less capable. She slips in classes, and she's beaten more, never really recovered from one before the next failure gets her more punishments. Nobody needs her, nobody wants her, and she knows this is how she'll die.
Then one day Sergi Mata, the boy from a few houses over, goes hunting. In the Wood. Apparently, she thinks, he wasn't beaten enough when he was young, because he comes back with a dead deer, scraped and torn arms and legs, and a story of killing a plant-fey's pet, twisting out of her grasping vines, and fleeing with its body. Which he brags about, in public, in front of the birds and the trees and anyone he can get his hands on. Silvia may be useless but she knows better than that, and so does the rest of the village. They see the trees start rearranging themselves in the night, and animals moving in unnatural patterns. This isn't a year they can afford even a little weevil-eaten grain, so the village puts together an offering, stretched though they'll be at the end. (Including no small amount of Sergi's blood, though he does survive it.) The offering does need to get to the forest, though, and they aren't going to send Sergi after he already angered the fey. Who else can they afford to lose?
When she hears what they're looking for, Silvia knows how this is going to turn out. She volunteers; she knows her place, and at least if it doesn't go badly, people won't be angry she made them wait. She goes in, carrying what they could afford to scrape together, milk and blood and a few precious drops of honey and all. When she reaches the flat stone that apparently used to mark the border of the forest, where some people said they used to trade with the fey, she looks around. The woman walking out of an oak tree and the wolf springing out at her are about what Silvia expected. When the wolf turns into a person — an elf, maybe? A nymph? Probably not a devil, those are the wrong stories — and, along with the woman, sits on the other side of the rock, she's surprised. When they take the offering peacefully and feed her a berry which fills her like she hasn't been full in years, when the still-open cuts on her back heal, she's baffled. When they give her back the basket, now holding ten of the same sort of berries, introduce themselves as Leafswirl and "the druid", and tell her where to find them, she almost breaks down in tears except for her years in school. She comes back, tells the villagers the fey are appeased as long as they stay careful, and eats the berries over the next week. (They need her now. She won't starve, she reminds herself. Eventually, two days before they were going to spoil, she slips a few berries into their dinner, and again the day after. Her family makes it to the harvest — not well, exactly, but better than average, definitely.)