Ruling 2403
It is unlawful for a man to marry women who are his maḥram,[11] such as his mother, sister, daughter, paternal aunt, maternal aunt, nieces, and mother-in-law.
—Ayatullah Sistani, Practical Laws of Islam, Marriage
1
A maḥram is a person one is never permitted to marry on account of being related to them in a particular way, such as being their parent or sibling.