Hello, I'm sorry to hear about your cat.
The most common cause of seizures in cats, especially older cats, is an intra-cranial lesion. Usually this is going to be a brain tumor. These are typically slow growing tumors and by the time they start causing seizures they've probably been there months. The only way to know for sure if this is the cause is by doing an MRI of the head. Plain radiographs will not image the brain.
As for toxins, the flea product may be responsible. Usually this happens hours after applying the flea product. It's also usually when owners put a dog flea product on a cat that causes the muscle tremors. These cats will respond to washing and treating with Methocarbamol and recover in 12-24 hours.
The fact that your other cat had seizures as well is not uncommon, although it is a bit of a coincidence. I would venture to guess that your other cat also had a brain tumor causing the signs- it's not uncommon for these to present late in life and lead to euthanasia.
The low potassium is likely secondary to the seizures. If the Phosphorous was also low, then I would ask about inhaler (Albuterol) toxicity. More common in dogs that chew on the inhaler, but it can be seen in cats.
I hope this helps. Let me know if you have any other questions.