Hindu Religion-the Gotra.

Gotra is lineage identification within caste that prohibits inter-marriage by virtue of our descent from a common ancestor. Gotra originally referred to the seven lineage sections of Maha-Rishis, who trace their derivation from Sapt-Rishi lineage, Atri, Bharadvaja, Bhrgu, Gotama, Kashyapa, Vasistha, and Vishvamitra. An eighth Gotra was added early on, the Agastya, named after the Seer intimately linked up with the spread of Vedic influence in southern India.

If you do not know your Gotra, Puranas say that you can name Kashyap Gotra as Sage Kashyap is the Originator of all human beings.  He hails all Gotras.

According to Hindu Marriage act 1955 Sec.5 (iv),  a marriage is voidable if the parties were within the degrees of prohibited relationship unless custom usage of a particular caste permits.

Therefore as a general rule:-

brother-sister,

uncle, maternal paternal with niece,

aunt, maternal paternal with nephew,

sons and daughters of brothers, sisters,

cannot marry with  each other.