Thank you all for the feedback! The discussion has been super useful and makes me hopeful we would win if this continues to escalate.
I wasn’t familiar with the term “augmented estate” but did some googling. Even if the IRA counted towards the augmented estate (which it seems unlikely it does here) she couldn’t claim it this way as they weren’t living together for a decade before he died, and she has significant assets of her own (a second house besides the one they owned jointly, two cars, and at least a few hundred thousand dollars she inherited from her mother).
She doesn’t need the money at all, just hates me and wants to harass me. Lovely, right? You can see why my dad wasn’t inclined to list her as a beneficiary.. now I just have to hope that dealing with her doesn’t cost so much that the account is drained. I will let you know how this shakes out.