I think [name_f]Helena[/name_f] [name_u]Harmony[/name_u] with the nickname [name_f]Hallie[/name_f] might be a good compromise! [name_f]Helena[/name_f] fits with your other daughters beautifully, but if your husband likes [name_f]Holly[/name_f], maybe he’ll like [name_f]Hallie[/name_f], too.
[name_f]Helena[/name_f] [name_u]Harmony[/name_u] is gorgeous! If he doesn’t approve, [name_f]Helena[/name_f] [name_u]Harper[/name_u] would be just as pretty 
Why not split the difference and go with [name_f]Helena[/name_f] [name_f]Holly[/name_f]? Is [name_f]Holly[/name_f] as a middle name too close to [name_f]Felicity[/name_f] [name_u]Hollis[/name_u]? Is [name_f]Holly[/name_f] as a FIRST name too close to sister mn [name_u]Hollis[/name_u]?
Good luck - [name_f]Helena[/name_f] is a GORGEOUS choice!