Am I sinful if my cousin sees something indecent on my phone that he takes from me, knowing that I have repented from seeing such things, and I saw something indecent yesterday because of him? So what is the solution for dealing with him?

You must advise your cousin and persuade him to stop watching haram images. Exert all possible means to convince him, such as giving him educational and admonitory tapes. Do not give him your mobile device if you suspect he will use it for disobedience. If he insists on his disobedience after your advice, then shun him for the sake of Allah, because Islamic law has commanded shunning those who persist in their rebellion and disobedience, especially when there is a fear of their influence on one's religion or morals. Allah Almighty says: "And do not incline toward those who do wrong, lest the Fire touch you." (Hud: 113). Shunning for more than three days is permissible for one whose conversation brings a deficiency to the addressed person in his religion or harm to him in his soul or worldly life.