From what I’ve been told, and what Apple alludes to on their website under the Q&A, no. It seems the devices will need to be signed into the same Apple Account, not just apart of the same family. But, if your family shares devices on the same Apple Account (ie, you and your wife share an iPad and Mac signed into your Apple Account/ID), then you could bundle those together. But not your wife’s phone and personal devices.
It feels like an incredible oversight. I’d even be happy if they made the ones shared by a family $25/month and lowered the savings a bit. Or increased the additional device costs. I’m not sure why they didn’t roll it out with that.
To me it seems this is fine for Macs where I can be signed in as the admin on various Macs and each used by wife, and kids, but wouldn't work for her phone or iPad or their watches. Of course we pretty much self insure everything except a tricked out Mac or AVP.