Boy marrying daughter of his maternal uncle or paternal uncle is evil as per shatapAtha brAhmaNa 1.8.3.5 & 1.8.3.6 :
5. Behind the juhu stands the sacrificer, and behind the upabhrit stands he who means evil to him: hereby, then, he brings the
sacrificer forward to the front (or east), and the one who means evil
him he drives back (or towards the west). Behind the juhu stands the
eater (enjoyer), and behind the upabhrit the one to be eaten (enjoyed): thus he now brings the eater (enjoyer) to the front, and
the one to be eaten (enjoyed) he drives back.
tadvā etat | samāna eva karmanvyākriyate tasmādu samānādeva puruṣādattā cādyaśca jāyate idaṃ hi caturthe puruṣe tṛtīye
saṃgacāmaha iti videvaṃ dīvyamānā jātyā āsata etasmādu tat
Thus the separation (of the eater and the eaten) is effected in one and the same act; and hence from one and the same man spring both the
enjoyer (the husband), and the one to be enjoyed (the wife): for now
kinsfolk (gatyah) live sporting and rejoicing together, saying, “In
the fourth man (or) third (i.e. generation) we unite.” And
this is so in accordance with that (separation of the spoons).
Above 4/3 rule considers line of descent. Together with line of ascent this would result in 7/5 rule as mentioned in texts like viShNu purANa book 3 ch. X :
Let a householder marry a maiden who is in kin at least five degrees remote from his mother, and seven from his father, with
the ceremonies enjoined by law
(i.e. 4+4-1 = 7 & 3+3-1 = 5)
yama dvitiya (Bhaubeej) festival, described in skanda purANa vaiShNava khanDa Kārttikamāsa-māhātmya ch. 11, considers daughter of maternal uncle as an alternative to sister:
25. In the absence of one’s own sister, one should go to the house of the sister of one’s father with respect and take food there.
If one has no younger sister, one shall go to the elder sister’s house. If she too does not exist, he should go to the abode of the
married daughter of his paternal uncle.
In her absence, to the house of mother’s sister or to that of the maternal uncle’s daughter. The order of precedence shall be fixed on the basis of the relationships as the children of co-wives or of
the same Gotra.
Marrying maternal uncle’s daughter was a curse to yadus & thus we have such marriages in case of krShna-nIlA, subhadrA-arjuna, pradyumna-rukmAvatI, aniruddha-rochanA :
padmapuraNa bhumI khanda ch.80
Yayāti, the lord of the earth, then cursed his son: “Since you have disobeyed (my) order, you, resembling a sinner, polluted by my curse,
enjoy a portion of your mother”
Penance for such marriages is given in skandapurANa-brahmakhanDa-dharmaraNyakhanDa-ch21
19. After marrying the daughter of the maternal uncle as well as a daughter of the same Gotra as that of his mother, and a girl of the
same Pravara (the man) should forsake her and perform Cāndrāyaṇa
rite.
kashyapa marrying daughters of dakSha is not an example of cousin marriage answered here
whypart of my question. – Ashish Srivastava Mar 06 '21 at 09:50