Hello, and congrats! You have so many wonderful names on this list. I’d describe your style as familiar, modern, and charming. First off, I’d avoid a unisex name with your sibset, as it might be assumed that you’ve welcomed another little brother. I think that for this reason, [name_u]Quinn[/name_u] would be better of in the middle. I also think that [name_u]Allison[/name_u] is a bit too close to [name_u]Carson[/name_u], even with the nickname. I also think that [name_f]Liv[/name_f] is a bit too short/incomplete with your sons’ names, so I wouldn’t use that.
To me, there are three perfect choices on your list: [name_f]Lily[/name_f], [name_f]Ivy[/name_f], and [name_f]Sophie[/name_f]. They feel very modern, feminine, and sweet, without being overly trendy. They fall right into the popularity zone of your boys’ names as well, helping them to fit in. While you can’t go wrong with any of them, I think my favorite for you is [name_f]Sophie[/name_f]. It bridges super modern [name_u]Carson[/name_u] with classic-yet-cool [name_m]Nicholas[/name_m], and has a nice length.