This isn't about you specifically, just a general observation about a lot of the conversations I've seen in the Writers' Corner, but...it seems like everybody is naming characters the way they would name a child, not how the character's parents would name a child. Does that make sense? Not everyone in the world is a name nerd, so when you name all the characters in your story uncommon, fancy, special names, it seems extremely unrealistic to me. I have a hard time believing that "Dianne" and "Robert" (or whatever) would have twin daughters named "Sienna" and "Cassia" or "Davina". And, while we name nerds might make a huge effort not to pick names for our children that are too matchy, many (if not most) of real twins I meet (as a substitute teacher) do have somewhat matchy names. "Normal" people apparently love twins to have names with the same initials. Also, think about the time period when the story is taking place and when the characters were born. (And take it with a grain of salt when a 14 year old tells you that _____ is an 80s name, because they weren't even born yet!)
I think a realistic "similar twin names" sibset would be Sienna and Savannah. (I don't care for either name personally, but I think they're realistic choices.) Sienna Claire works well as Claire is a fairly common middle name. I would go with something like Savannah Rose, or another "filler" middle name, because that's how people name their kids in the real world.
(But, seriously, how important is it for story characters to have middle names anyway?)
I thought of Lianne or Leanne.
I would say the first had better be spelt Liane
that can keep the two sisters's names from being freakishly parallel