Rosemarie Sienna.
I keep reading this combo as
Rosemary. I guess subconsciously I prefer that!
Rosemarie Sienna is a nice combo, though it's NMS. I don't love
Sienna, but I think it sounds better than
Sophia and agree that 5-10 years old is too young to honor, even though the intention is very, very sweet. I also think it's great that
Sienna holds such a significance for you and your SO.
If you wanted to try and honor your sister, though, you could try to find another name that means the same thing as
Sophia, which is "wisdom" in Greek. I think that could be a subtle way of honoring her without being too in your face about it.
http://www.behindthename.com/php/search.php?nmd=mdu&terms=wisdom