Sebastian is my fave boys’ name! And I would personally go with Benedict for the second twin.
It matches so well with both the other boys, has the “blessed” meaning, and avoids some of the split second stumbles for some with pronouncing Benoit.
I’d go Benny for Benoit but you could go further afield with Benedict: Ned, Nix, Ted, Dex, Beck, and even Kit!
(With Benoit you could maybe go Toby, Noa, Neo, Waby, Bo.)
Beckett would be Beck or Kit for me! Bash and Beck did make me think Posh & Becks 