It is unlikely for the speakers sister to be the best if she planned the birthday party on a day that is inconvenient for the person, instead she is selfish and inconsiderate.
Consider the statement "Even after knowing my schedule my sister planned a birthday party for our mother on a day thats inconvenient for me.". Is the statement "Isn't my sister the best for purposefully planning a birthday party for our mother on a day that's inconvenient for me inspite of knowing my schedule." false? Yes/no/unknown