Is the dissemination of pictures of a sister who previously did not wear the hijab, by her Christian brother's wife, considered ongoing sins for her after her repentance, and how can this act be addressed?

What this woman is doing by publishing your photos from before you wore the hijab is an evil act (munkarat), and you must advise her to stop, and seek Allah's help and supplicate to Him by saying something like: "O Allah, we place You against their throats, and we seek refuge in You from their evils."

You should also advise your brother regarding his wife's actions, and seek help from your parents or the wise ones among your relatives. If she does not stop, you can threaten to report the matter to the responsible authorities.

If she persists despite your disapproval, then you bear no sin; rather, it is a trial, and enduring it with patience raises one's ranks and expiates sins.