I like it! Hathaway [name_f]Pearl[/name_f] has a lovely flow to it. I’m quite taken by ay ending names lately, I also like [name_f]Holliday[/name_f], Halliday and [name_m]Calloway[/name_m].
From your lists, my favourites for a sibling to Hathaway [name_f]Pearl[/name_f] would be:
I generally like surnames as first names, but Hathaway just doesn’t sound like a given name to me. One of my friends has the last name Hathaway, so I think of him first. We sometimes call him by his last name, but that’s more a way of teasing him. If it were to be given name, then it’s all male to me.
I rather like it! It has a soft, warm sound and a nice cadence, plus a great literary connection. Hathaway [name_f]Pearl[/name_f] is very pretty and interesting — a nice balance between unique and grounded.
From your list, [name_u]Flannery[/name_u] [name_f]Mabel[/name_f] feels spot-on for a sister, with the same literary surname + sweet vintage middle thing going on.
[name_m]Whittaker[/name_m] [name_m]Hayes[/name_m] and [name_u]Sullivan[/name_u] [name_m]Arrow[/name_m] feel like the best fit for a brother, but I’d maybe go more traditional in the middle to match [name_f]Pearl[/name_f] better. [name_m]Whittaker[/name_m] [name_m]Ives[/name_m] and [name_u]Sullivan[/name_u] [name_m]Arthur[/name_m], for example.
I find it gentle and lulling too but I’m from a nicknaming country so you’d be dealing with Hath or Hatha (Yoga) which aren’t great. [name_f]Hattie[/name_f] might work, but seems far from Hathaway. I just thought now: kid could be teased - go away. Maybe that’s a risk with most NB names! I don’t know, I want to get on board as can genuinely see the appeal but would never actually use it myself.
I am not a fan of surnames as first names, and Hathaway just doesn’t have any appeal to me. It looks like you are trying too hard to come up with a new surname.