"asapiNDA cha yA mAtuH asagotrA cha yApituH sA dvijAnAM prashastA strI dArakarmaNyamaithunI"
- Manu Smriti
"asamAnArSheyAmaspRShTamaithunAM avaravayasIM bhAryAM vindeta"
"mAtRtaH panchamAdUrdhvaM pitRtaH saptamAditi"
As per the above, a boy should not marry a girl who is sapiNDa that is descendants of the same womb or a girl who is of the same gotra as the his father. However there's no bar on a 7th gen boy marrying a 5th gen girl of the same lineage. A boy marrying his paternal aunt's daughter or a maternal uncle's daughter therefore doesn't contravene these provisions and hence permitted. However, we do see girls marrying own maternal uncles in south I don't know how.
other provisions in regard to marriage is that the girl should not be elder to the boy and should not have had pre-marital relationship.
My doubt is why is same thing not practiced in North India?
- So for a background, In North paternal aunt's daughter and maternal uncle's daughter are treated as sisters even though their gotra are different considering their father's gotra.