If you’re both filing taxes as single, you should consider doing the paper work, and get married, I think. That’s exactly the kind of thing that reaches back and botches things up. Usually at some crucial juncture. Something to think about.
And I say this in year 38 of a relationship without ‘the benefits of marriage’, as they used to say. We have long filed taxes as common law partners, (it’s all recognized here!) And we have all our ducks in a row with power of attorney, for medical decisions, finances, both our names are on all the accounts, etc, etc.
No chance I’d have been okay to be deemed simply, ‘next of kin’, in any of that paperwork. That seems like asking for trouble. Mine all specifies ‘common law wife’.
You should consult an attorney, in my opinion. Good Luck!