Can dreams be relied upon when taking means (actions), and does Satan exploit visions to harm a Muslim or prevent him from good? And is what the questioner sees in his dream about marrying his cousin from Satan's beautification to deter him from what is better? If so, how should he deal with a good vision?

What a sleeper sees in their dream may be a vision (ru'ya) from the Most Merciful, or a dream (hulm) from Satan, or a self-discourse. One should not rely on dream visions, but what amongst them contains good can be a source of comfort. If a Muslim wishes to marry, they should seek a religious and good woman, as the the Prophet, peace and blessings be upon him, said: "So win the one who is religious, you will prosper." Then, they should seek Allah's guidance (istikharah). If your cousin is good and has repented from listening to music, and her father desires your marriage to her, then proceed with the marriage along with performing istikharah.