Answer: I tend to side with your experienced sister-in-law. Corn is a heavy feeder and will respond to a side-dressing of blood mel or a diluted fish-based fertilizer. The should be fed when the stalks are 6" tall, and again when they're about knee-high. If you missed a feeding, it might explain why the stalks are spindly. Feed now, and water deeply once a week throughout the season.
Bulbing onions will indicate they're ready for harvest when the tops yellow at the end of the growing season. I hope you separated the plants when you planted and didn't just plop the whole tray into one planting hole! Onions develop green leaves for each ring they develop in the bulb. The more leaves, the larger the bulb at the end of the season. (Or at least that's been my experience.)
Hope you have a great harvest this autumn!
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