References please.
Neither Hebrew nor Aramaic (the language spoken by Christ and his disciples) had a special word meaning "cousin," speakers of those languages could use either the word for "brother" or a circumlocution, such as "the son of my uncle." But circumlocutions are clumsy, so the Jews often used "brother."
The writers of the New Testament were brought up using the Aramaic equivalent of "brothers" to mean both cousins and sons of the same father—plus other relatives and even non-relatives. When they wrote in Greek, they did the same thing the translators of the Septuagint did. (The Septuagint was the Greek version of the Hebrew Bible; it was translated by Hellenistic Jews a century or two before Christ’s birth and was the version of the Bible from which most of the Old Testament quotations found in the New Testament are taken.) In the Septuagint the Hebrew word that includes both brothers and cousins was translated as adelphos, which in Greek usually has the narrow meaning that the English "brother" has. Unlike Hebrew or Aramaic, Greek has a separate word for cousin, anepsios, but the translators of the Septuagint used adelphos, even for true cousins.
New Testament:
Cousin
Luk 1:36 And, behold, thy cousin Elisabeth, she hath also conceived a son in her old age: and this is the sixth month with her, who was called barren.
Luk 1:58 And her neighbours and her cousins heard how the Lord had shewed great mercy upon her; and they rejoiced with her.
Col 4:10 Aristarchus, my fellow prisoner, sends you his greetings; and {also} Barnabas’s cousin Mark (about whom you received instructions; if he comes to you, welcome him);
Kin
Mar 6:4 But Jesus said unto them, A prophet is not without honour, but in his own country, and among his own kin, and in his own house.
Luk 2:44 But they, supposing him to have been in the company, went a day’s journey; and they sought him among [their] kinsfolk and acquaintance.
Luk 14:12 Then said he also to him that bade him, When thou makest a dinner or a supper, call not thy friends, nor thy brethren, neither thy kinsmen, nor [thy] rich neighbours; lest they also bid thee again, and a recompence be made thee.
Luk 21:16 And ye shall be betrayed both by parents, and brethren, and kinsfolks, and friends; and [some] of you shall they cause to be put to death.
Jhn 18:26 One of the servants of the high priest, being [his] kinsman whose ear Peter cut off, saith, Did not I see thee in the garden with him?
Act 10:24 And the morrow after they entered into Caesarea. And Cornelius waited for them, and had called together his kinsmen and near friends.
Rom 9:3 For I could wish that myself were accursed from Christ for my brethren, my kinsmen according to the flesh:
Rom 16:7 Salute Andronicus and Junia, my kinsmen, and my fellowprisoners, who are of note among the apostles, who also were in Christ before me.
Rom 16:11 Salute Herodion my kinsman. Greet them that be of the [household] of Narcissus, which are in the Lord.
Rom 16:21 Timotheus my workfellow, and Lucius, and Jason, and Sosipater, my kinsmen, salute you.
For the Aramaic, find an online Aramaic lexicon and search for cousin. I found one a while back, but don’t feel like researching it again.
As you can see, the NT writers did know how to refer to someone as cousin or kin, instead of brother.