[name_m]Bennett[/name_m] & [name_m]Barrett[/name_m] are definitely pretty similar names, although the differing starting sounds help prevent too much confusion (ex [name_m]Bennett[/name_m] & [name_m]Benji[/name_m] would be too close for me). I’d ask yourself how often you see your nephew, and if your sister/brother would be at all miffed at the similarity between the names.
Would [name_m]Barrett[/name_m] regularly be going by a nickname, like [name_m]Bear[/name_m]? That might also help to separate the names a bit more!
From your list, I think [name_m]Garrett[/name_m] is a great alternative for y’all! Also loving [name_m]Grant[/name_m], [name_m]Warren[/name_m], & [name_m]Wyatt[/name_m]! Those would all flow great with [name_f]Mckinley[/name_f] in the middle spot as well.
A few other suggestions:
[name_m]Emmett[/name_m]
[name_m]Alcott[/name_m]
[name_m]Elliott[/name_m]
[name_m]Marshall[/name_m]
[name_m]Reid[/name_m]
[name_m]Jude[/name_m]
→ [name_m]Benton[/name_m] works as a middle with [name_m]Reid[/name_m], [name_m]Jude[/name_m], & [name_m]Marshall[/name_m]! I think he flows better with a one syllable first name and/or names that don’t contain a strong T sound