Is one considered a sinner if they help a relative conceal their sin, such as by deleting the browsing history on the browser through which their relative accesses pornographic websites, while knowing that their relative continues to do so?

The basic principle is to think well of a Muslim and not to accuse the innocent without proof. The pictures might have been unintentional. There is no harm in you suggesting to him (to take measures) to protect his daughters. If it is highly probable that he is afflicted (with a trial), you can gift him a book or a lecture that encourages mindfulness of Allah, or leave a message about lowering the gaze without his knowledge, while providing permissible and beneficial websites, and supplicate for his guidance.