This sounds like an amazing trip!
My thoughts if I was planning it for my wife:
Location matters a ton Santorini and saving a couple thousand dollars isn't worth it if you're far from the restaurants, views, and activities. I'd pick the area first (likely Oia or Imerovigli) then look at villas in that area.
I'd consider ditching Mykonos. It can be very busy and party-focused in July, and there's enough overlap with Santorini that I'd rather use that portion of the trip to experience a different side of Greece.
I'd probably structure it something like:
5 nights: Santorini - with your family
The iconic Greece portion: caldera views, sunset, sailing, wineries, great restaurants, etc.
5 nights: Naxos or Paros - with her girlfriends
This is where I'd look for somewhere fun and social without being a full-on party destination. If they want restaurants, shopping, beach clubs and a lively town, I'd lean toward Paros/Naoussa. If they want something a little more relaxed, with great beaches, local food and a less polished feel, I'd consider Naxos.
5 nights: Paros, Naxos, or Milos - just the two of you
I'd actually change the accommodation style. After hosting family and friends for 10 days, I'd put the money into a beautiful hotel or suite rather than another villa. This should be the romantic, slower-paced portion, great room, pool, beach, a private boat day, etc.
Logistically, I'd try to move in one direction. Ferries connect the islands, so you can avoid adding a bunch of flights and just make each island change a dedicated travel day. I'd also arrange private transfers on either end so she isn't dealing with luggage, ferry terminals and taxis.
I also don't hate doing it in the reverse (not to confuse you). You'd save Santorini for the finale and splurge on a beautiful caldera-view hotel for the two of you, which could make the last five nights feel especially special for her 50th. Naxos can be great with family…relaxed, active island with great beaches, restaurants, villages and plenty for adult kids to do.
Happy to answer questions or help with the trip if you're looking for it!
Luxury & Adventure Family Travel Agent
Personal Travel Concierge
Europe • Africa • S. America
www.voce-travel.com
Kyle.regitz@fora.travel