What do you think goes the best? Olivia or Livia?

I definitely think Olivia Sophie fits in best with your other daughters' names. Livia sounds more like a nickname than a full name, and it doesn't fit with Isla, Violet and Sienna - it seems a little out of place, whereas Olivia fits perfectly. Also, if you use the name Olivia, you still have the possibility of using Livia as a nickname as well as Livy - it leaves your daughter more choices for the future. Personally I would spell Livy as Livvie, but I think Livi or Livy is the cutest spelling from your list. :]