I quite like Sienna but can’t imagine that would be picked. I do wonder whether Isla or Lily will be in the running though. Lily, as well as a whole host of late Victorian names, have become very popular in the UK over the last decade.
I wouldn't be overly surprised by Lily. I don't think Isla is a possibility simply because Peter Philips has a daughter named Isla. I think they'd avoid using a name that closely related. Granted, I don't believe middle names should follow the same convention. For example, I don't believe they'll use something like Charlotte, Louise, Isla, Savannah, Mia, or Lena simply because close relatives have those first names. However, they shouldn't and won't avoid something like a middle name of Diana or Elizabeth simply because she might share those with a cousin. I'm not sure I'm explaining it well but I feel like middle names are entirely fine to share while first names, at least for those closely related, should be unique or as unique as possible.