Note how in Luke 14 & 21, Adelphos and Suggenes are in the same list. In other words, they are different from one another.
Lu 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.
Lu 1:58 And her neighbours and her cousins heard how the Lord had shewed great mercy upon her; and they rejoiced with her.
Lu 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.
Lu 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.
Lu 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.
Yes, and also anepsios for nephew. However, the usage is that adelphos is a generic term of relatives of roughly the same age, when the specific narrow term cannot be applied to everyone in the group, or for some other reason not advisable. We see that expansive usage throughout the scripture and it is still common in the Middle East and Greece today.
One simple reason to use the generic term is that Jesus liked to use the word "brother" to refer to His disciples. Imagine the gospel where strangers are called warmly "brothers" whereas the flesh and blood relatives coolly "cousins". It would have been outright confusing.