In pharmacy, you could truly be written up for just about anything. Everyone makes mistakes and it's all about what I call the 'halo effect'. If they like you, they will overlook most of your mistakes, if they don't like you, even non-mistakes will be the most major mistakes. I've worked with people who have made some jaw-dropping errors, and they were treated like royalty. If I missed a period in the sig, it's the greatest disaster since three mile island.
To the original poster: it may take the rest of your natural life to fight this as wrongful termination, but you might just win in the end... It is very questionable if a boss's family member is your 'replacement'. Definitely a case I would want to throw in front of a jury if I were a lawyer, and they would probably settle before it ever did...
