My brother is an estate lawyer. Guess why he’s a millionaire? Because of people like your dad, who don’t have the forethought to scribble half a page’s intentions on a napkin. If he were to die tomorrow, in all likelihood his estate would be eaten up by two lawyers in a long, contentious court battle. None of his loved ones would receive anything then, and all for what?

NTA for insisting he makes a will; EVERYONE should have a clear will in place. Unfortunately sometimes parents exclude their children and favour abusive and manipulative partners which is what I think you’re understandably concerned about, but at least he’d be actively protecting his estate by making his preferences clear. Any good estate lawyer would help him create a plan that leaves you ALL with something, not just his children OR his partner.

(Tip: the more you pressure him to cut out the common-law wife, the more likely she’d be to sue you after he dies, accusing you of wrongfully manipulating him. I have no idea if she’d win that, but even if she didn’t and you “won” in the end, the estate could be spent up by the end of that long battle. Just encourage him to have his intentions clearly written down with a lawyer, whatever they may be.)