I actually think [name]Cassidy[/name] improves as a sister to [name]Sienna[/name] but they won’t always be known as “[name]Sienna[/name] and [name]Cassidy[/name]” so I guess I’m back to not being sure if [name]Cassidy[/name] is a boy or girl. I prefer it on a girl because of the strong [name]Cassie[/name] sound but it’s such a cowboy kind of name that it seems to fit better on a boy. [name]Sienna[/name] has the same western feel but is so obviously feminine, I guess it would make me wonder more if [name]Cassidy[/name] was her brother. I also think that [name]Si[/name] [name]Si[/name] and [name]Cassidy[/name] have too much sss sound and [name]Si[/name] [name]Si[/name] limits what you might call [name]Cassidy[/name] as a nickname (or makes them too similar).
[name]Cassidy[/name] is very 90s for me, I think because I immediately think of [name]Regis[/name] and [name]Kathie[/name] [name]Lee[/name], when [name]Kathie[/name] [name]Lee[/name] would always talk about her kids [name]Cody[/name] and [name]Cassidy[/name]. (Clearly I watched too much morning tv in the 90s, haha)
I do much prefer [name]Cassandra[/name], [name]Cassie[/name] is an adorable nn. [name]Sienna[/name] and [name]Cassidy[/name] do make more sense together than [name]Sienna[/name] and [name]Cassandra[/name] though. [name]Sienna[/name] and [name]Cassia[/name] are nice.
My boyfriend’s last name is [name]Cassidy[/name], and I love it as a last name. I hope to take it as my own last name someday. 
But as a first name, I’m not a fan. I’m not into the surname as a first name trend. I also love [name]Cassandra[/name]. =)
Honestly, [name]Cassidy[/name] is dated and butch to me. [name]Cassia[/name], [name]Cassandra[/name], or just [name]Cassie[/name] would be infinitely better.
I like [name]Cassidy[/name], but I prefer [name]Cassandra[/name].
Maybe [name]Cassidy[/name] [name]Eve[/name], [name]Cassidy[/name] [name]Noelle[/name], [name]Cassidy[/name] [name]Elise[/name], [name]Cassidy[/name] [name]Jane[/name], or [name]Cassidy[/name] [name]Quinn[/name]
It’s just my personal opinion is all. Wasn’t trying to be amusing or offensive. I realize the letters are there, but not the actual sound of “cass”. It’s pronounced [name]Ca[/name]-ssandra, not [name]Cass[/name]-andra. Or am I mistaken?
[name]Cassady[/name] (yes, with two As) is one of my favorite names for a boy. DH & I love the nickname [name]Cass[/name]. We talked about using it if our LO is a girl but the NN [name]Cassie[/name] turned me off. I think [name]Cass[/name] is a sweet & spunky nn for a little girl but am too worried that she’d automatically get called [name]Cassie[/name].
I agree that a feminine mn is the way to go.
I don’t see how it could be a boy’s name, it seems very feminine to me. I have a good friend named [name]Cassidy[/name] [name]Nicole[/name].
edit: definitely prefer original “[name]Cassidy[/name]” spelling
I really like the nickname “edie” for [name]Cassidy[/name] as well!
I think I agree that it’s a truly unisex name and I love it on both genders. Very spunky name!
I prefer it on a boy but I still think it’s a nice choice for a girl too 
I’m not particularly fond of [name_f]Cassie[/name_f] though, I’d use [name_m]Cas[/name_m]/[name_u]Cass[/name_u] as a nickname myself