Expecting little girl, big brother [name_u]Elliot[/name_u] [name_u]Michael[/name_u]
Middle name will be [name_f]Catherine[/name_f] after my grandmother, last name begins with R and three syllables
Looking for thoughts on first names…
[name_f]Rose[/name_f] [name_f]Catherine[/name_f]
[name_f]Mary[/name_f] [name_f]Catherine[/name_f]
[name_f]Isla[/name_f] [name_f]Catherine[/name_f]
[name_f]Julia[/name_f] [name_f]Catherine[/name_f]
My favorites from your list are [name_f]Rose[/name_f] [name_f]Catherine[/name_f], [name_f]Mary[/name_f] [name_f]Catherine[/name_f] and [name_f]Isla[/name_f] [name_f]Catherine[/name_f]. As much as I want to like [name_f]Julia[/name_f] [name_f]Catherine[/name_f], something sounds off about it to me. More first names that may work for you are:
[name_f]Abigail[/name_f] [name_f]Catherine[/name_f]
[name_f]Amelia[/name_f] [name_f]Catherine[/name_f]
[name_f]Anabelle[/name_f] [name_f]Catherine[/name_f]
[name_f]Beatrice[/name_f] [name_f]Catherine[/name_f]
[name_f]Elizabeth[/name_f] [name_f]Catherine[/name_f]
[name_f]Emily[/name_f] [name_f]Catherine[/name_f]
[name_f]Isabelle[/name_f] [name_f]Catherine[/name_f]
[name_f]Ivy[/name_f] [name_f]Catherine[/name_f]
[name_f]Lily[/name_f] [name_f]Catherine[/name_f]
[name_f]Phoebe[/name_f] [name_f]Catherine[/name_f]
[name_u]Sage[/name_u] [name_f]Catherine[/name_f]
[name_f]Sophia[/name_f] [name_f]Catherine[/name_f]
[name_f]Willa[/name_f] [name_f]Catherine[/name_f]
[name_f]Willow[/name_f] [name_f]Catherine[/name_f]
[name_f]Sunniva[/name_f] [name_f]Catherine[/name_f]
[name_f]Yara[/name_f] [name_f]Catherine[/name_f]
My favorite is [name_f]Mary[/name_f] [name_f]Catherine[/name_f]! I think [name_f]Mary[/name_f] is really unexpected nowadays, and it’s a strong, traditional choice with the cute nickname [name_f]Mae[/name_f]. My second favorite is [name_f]Rose[/name_f] [name_f]Catherine[/name_f]. You could also combine [name_f]Rose[/name_f] and [name_f]Mary[/name_f] into [name_f]Rosemary[/name_f], if that’s your style.
[name_m]Hi[/name_m] there.
I love [name_u]Elliot[/name_u]! Great job there.
Here are my thoughts on the list for a little sister:
[name_f]Rose[/name_f] [name_f]Catherine[/name_f]-This is gorgeous. Very classic & simple with the sweet nn of [name_f]Rosie[/name_f]. Is it tongue twister-y with your ln?
[name_f]Mary[/name_f] [name_f]Catherine[/name_f]-This is another simple classic. I like [name_f]Rose[/name_f] more, but [name_f]Mary[/name_f] is sort of unexpected these days. I’m not sure if you’re a nn person, but I mainly love [name_f]Mary[/name_f] for the nns of [name_f]Maisy[/name_f] & [name_f]Molly[/name_f].
[name_f]Isla[/name_f] [name_f]Catherine[/name_f]-This is quite nice. Very over done though. I meet a lot of little Islas. I’m not sure how much that bothers you. It’s a pretty name though that flows well w/[name_f]Catherine[/name_f] & [name_u]Elliot[/name_u].
[name_f]Julia[/name_f] [name_f]Catherine[/name_f]-This is nice. Seems the most compatible style w/[name_u]Elliot[/name_u].
[name_f]Hope[/name_f] this helps. 
[name_f]Rose[/name_f] [name_f]Catherine[/name_f], [name_f]Julia[/name_f] [name_f]Catherine[/name_f], and [name_f]Isla[/name_f] [name_f]Catherine[/name_f] are all gorgeous! [name_f]Mary[/name_f] [name_f]Catherine[/name_f] feels very Catholic to me (very common pairing where I grew up) – not a bad thing, just throwing it out there. I do love [name_f]Mary[/name_f], though, for being such an unexpected and classic choice these days!
Over all, my favorite would be [name_f]Rose[/name_f] [name_f]Catherine[/name_f], closely followed by [name_f]Julia[/name_f] [name_f]Catherine[/name_f]. [name_f]Isla[/name_f] feels a little similar in sound too [name_u]Elliot[/name_u], imo, but perhaps that’s something you prefer.
[name_f]Hope[/name_f] this could help!
I like [name_f]Isla[/name_f] best!
All of the combinations are very pretty. My favorite is [name_f]Julia[/name_f] [name_f]Catherine[/name_f]. I had a great-aunt (born around 1915) named [name_f]Julia[/name_f] [name_f]Catherine[/name_f], and I always thought her name was really pretty. My second favorite is [name_f]Rose[/name_f] [name_f]Catherine[/name_f], followed by [name_f]Mary[/name_f] [name_f]Catherine[/name_f].
[name_f]Etta[/name_f] [name_f]Catherine[/name_f]
[name_f]Ella[/name_f] [name_f]Catherine[/name_f]
[name_f]Lissa[/name_f] [name_f]Catherine[/name_f]
[name_f]Rita[/name_f] [name_f]Catherine[/name_f]
[name_f]Monica[/name_f] [name_f]Catherine[/name_f]
[name_f]Sara[/name_f] [name_f]Catherine[/name_f]
[name_f]Ayla[/name_f] [name_f]Catherine[/name_f]
[name_f]Mia[/name_f] [name_f]Catherine[/name_f]
My name is [name_f]Julia[/name_f] [name_f]Catherine[/name_f] and I’ve been very happy with it! Personally, [name_f]Mary[/name_f] and [name_f]Rose[/name_f] are also high on my list and I’ve considered passing down [name_f]Catherine[/name_f] as a MN with either of them too. I think all three are wonderful names and combinations. Not a big fan of [name_f]Isla[/name_f].