"the day she agreed to give him the life estate, making her ownership tenants in common"

No, that is not what creates "tenants in common."  For that she would have had to give him half the property outright, and then a life estate in the half she kept.  And his estate would still own the other half, with presumably the widow claiming some spousal rights to it.  

She could have put the property in joint tenancy with right of survivorship, but then it would be a race to the graveyard and Bro might have outlived her.  The estate planning attorney (I'm one of those, too) is simplifying things too much by calling life estate "a form of joint ownership."  It's more like, "a way to accomplish some of the benefits of joint ownership, without having to create one."