Lots of differing opinions here.
If the Rio you used at the school is one of the older Start/later Go based models and not one of the
newer narrower/longer Rios that have much better glide in light winds then perhaps it would be good for you to go and take a few more lessons to try out some different boards.
Getting an older Go board (without a center board or center fin) presents problems (not insurmountable) with staying upwind and many of the basic transition moves as it does not "steer" in the same way as the Rio with a centerboard.
Something to consider when learning or teaching beginners.
The volume (float) of the Rio S suits your girlfriend's weight more than it suits your weight.
Have you actually sailed one with a small ~5.2 m2 rig in < 3 BFT of wind?
At your 72 Kg. weight, you could easily sail the Rio S with a 7.0-7.5 m2 rig and begin to get some
performance with the centerboard retracted.
With a ~ 5.0 rig in < 3 BFT......not so fun for a sailor of your weight.
For your girl friend and your kids (how old are they and how long until the will want to try windsurfing?)
a smaller lighter rig is going to be the key to their success.
Put them on a <60% carbon mast on a wave or freeride 5.0 m2 sail designed and built for higher winds, and they will probably give up on windsurfing because the rig is so heavy.
If you have more wind where you plan to sail, then the 5.0 might work for you, but would be totally too big for the GF and kids.
As suggested, you will most likely need/want another board for your skill level, so the Rio S makes the most sense for the girl friend and kids.
You, on the other hand would do alot better on the Rio M with your current skill set, but will soon want a shortboard and larger sails.
A tough choice indeed!