Dharma is commonly translated as 'law', 'vocation', 'religion' or 'code of conduct', when in fact the word literally means doing what you are born to do. From the root of the word dhr - 'to uphold'.
The opposite of that is Adharma (against dharma), the acts which imped or pervert the current of your existence.
Well before the concept of Karma firmly took the central stage in the pantheon of Vedic, Buddhist, Hindu thought it was known as Adharma 'that which is opposite of Dharma'.
After all we are not punished for our actions, but by our actions. Yet Karma did not become sin until the lawgivers made it into a sin. Until than it was Adharma.
Dharma is the universal law which confirms the nature of acts which are inherent in the nature of things. The Dharma of the Sun is to shine. Full stop ! Just like the dharma of the Moon is to shower the rain on Earth.
Adharma is not a sin either, it is simply non-confirmity with the nature of things. A crime against harmony. It is from there that the nature of the philosophy of Karma proceeds.
Whence Dharma or doing what we are born to do is, ultimately speaking, rests on being in harmony with the Natural Law. When the Natural Law is being violated it is our Dharma to uphold it from within regardless of our individual Karma. The very essence of Sanatana-dharma.
Jai Guru Dev
