Union with a Mahram
Scholars have said that if one dreams of uniting with a deceased mahram (a relative with whom marriage is forbidden), it may indicate that the dreamer will benefit the deceased through charity or prayer, or that their descendants will receive good from the deceased. It has also been said that it might signify committing a forbidden act.
Per Ibn Sirin's Muntakhab al-Kalam fi Tafsir al-Ahlam