In Udāna 3.2, Venerable Nanda is said to be a close relation to the Buddha.
bhagavato bhātā mātucchāputto
The Buddha’s brother, son of his maternal aunt
After the death of the Buddha’s mother, Suddhodana remarried to Māyā’s sister, Mahāpajāpati. Nanda is presumably her son. That would make him both the Buddha’s half-brother (they have the same father) and cousin (son of his aunt).
The question is, in translation, should we render the Pali literally, or specify the relation in more comprehensible terms. Most translations seem to take the literal route. A justification for this might be that since we know so little about the Buddha’s relations, we should take care to render the text without making assumptions.
Still, it seems odd to me to just say “brother” here. Perhaps:
Venerable Nanda, who is both the Buddha’s half-brother and maternal cousin