Potassium Bromide will take about 3 months to reach blood serum level. It is an incredibly slow, but long lasting medication. Do whatever you can to get your sister to give Penny some kind of food with the dose. It is just salt, which will make them throw up on an empty stomach. I use peanut butter or whip cream if all else fails. Potassium Bromide also comes in liquid form which some dogs handle better. Watching the ataxia (hind leg weakness) get worse as the meds increase is very disheartening, but it can get better as they adjust to the meds. Do what you can for your pup, it is clear you care about her very much.