Your boys have wonderful names! I am especially partial to [name_m]Colin[/name_m] since it is the name of my younger brother. From your list, my first choice is [name_f]Margot[/name_f], followed by [name_f]Violet[/name_f]. I also like the combinations [name_u]Emerson[/name_u] [name_f]Violet[/name_f] and [name_f]Margo[/name_f] [name_f]Sophie[/name_f] or [name_f]Sophie[/name_f] [name_f]Margot[/name_f].
My suggestions:
[name_f]Adeline[/name_f]: [name_m]Colin[/name_m] and [name_m]Benjamin[/name_m] are both popular, classic names with history and gravity. [name_f]Adeline[/name_f] fits this style to a t, and it suits the vintage feel of [name_f]Margot[/name_f], [name_f]Violet[/name_f], and [name_f]Sophie[/name_f].
[name_f]Teresa[/name_f]/[name_f]Theresa[/name_f]: I feel bad recommending my own middle name, but I think it would work very well with [name_m]Colin[/name_m] and [name_m]Benjamin[/name_m]. It is a serious name with lots of excellent namesakes, and is vaguely British like [name_m]Colin[/name_m].
[name_f]Emmeline[/name_f]: If you are looking for an “E” name, I think this could be just about right. It has both the “[name_f]Em[/name_f]-” of [name_u]Emerson[/name_u] and the “-el” of Elliotte. This is another vintage classic, and matches the style of [name_m]Colin[/name_m] and [name_m]Benjamin[/name_m]. It is a beautiful, strong name.
[name_u]Rowan[/name_u]: Tailored, like [name_m]Colin[/name_m], but subtly containing some of the sounds of [name_m]Benjamin[/name_m]. This name has mystique and substance, and is unisex like [name_u]Elliot[/name_u], [name_u]Emerson[/name_u], and [name_u]Everly[/name_u].
[name_f]Phoebe[/name_f]: Such a bright, beautiful feminine name. It’s beautiful like [name_f]Sophie[/name_f] but with an unusual and vibrant twist, like [name_u]Emerson[/name_u]. I think [name_m]Benjamin[/name_m], [name_m]Colin[/name_m], and [name_f]Phoebe[/name_f] make great siblings.
For combo ideas from this list, [name_f]Theresa[/name_f] [name_f]Emmeline[/name_f] or [name_f]Phoebe[/name_f] [name_f]Emmeline[/name_f] are beautiful, and as is [name_f]Adeline[/name_f] [name_u]Rowan[/name_u].
[name_f]Hope[/name_f] this helped, and best of luck!