You can definitely share with another family. Just link together on MM+ and when you go into photopass you'll see everyone's photos. The only pain in the rear is that either everyone needs to go through and download their own photos separately, or you can each download the whole thing then go through and delete what you don't want.
One thing to be aware of is that there usually aren't PP photographers at character meals. So if most of your character interactions are at meals, you'll be depending on your own photographs. And sometimes we feel like we spend more time than we should tracking down photographers in the parks, to make sure we get value out of our MM. But on the other hand, it makes it so easy to get whole-family shots that we normally wouldn't have. And the magic shots really are cool.
DC1 -- 2005 DD -- 2009 DS -- 2011