Situation is expected.
Twist is unexpected. As always, the difference is blurred; you can set up a twist, foreshadow it, build up to it, and smart readers will foresee it, so for them it will be more of situation. But unforseen situation is a twist, something that changes the game. Forseen twist is a situation, something inevitable.
Both can create a choice, or become no-choice, but while one goes with the flow of the story, the other changes the flow.
Ralph can still refuse to write that will, but that's merely a denial, that doesn't change the outcome. Since we know the wife will die, no matter what Ralph does, the ultimate end will be the same. We can see the struggle, we can see small victories or even deeper loss, but the end will be just as the situation dictates it.
Now a twist would be if a cure for Ralph's wife is discovered. It may be readily available, or it may require a major challenge to obtain. It changes the game though - from waiting for the inevitable and a combat lost before it started, to a game that offers a chance of victory.
Let me give you another example of these. The game called "Summoner" creates one huge twist.
The protagonist, Joseph, is a commoner in a medieval country, who has a tattoo on his hand, that grants his unique talent - or curse, depending how you look at it. He is able to summon demons. Which may, or may not do his bidding. In his childhood that led to a demon burning his home village. The demons now released threaten the realm.
The game revolves around Joseph gathering magic rings that can be used to control the demons. Gathering a ragtag team, following guidance of a mysterious King's advisor Jago, Joseph is to gather the rings and destroy them in Gods' Forge, in order to banish the demons. There's a side-thread of a hostile neighbor kingdom preparing for a war, world torn by conflicting forces, armies gathering, and in this climate we face the situation: gather the rings or face doom.
Finally, Joseph with the complete set of rings reaches the gods' forge, and immerses his ring-clad hand in the fires of the forge.
And now comes the twist. Jago was possessed by one of the demons. This was all a ploy. Not only destruction of the rings releases any restraints that held the demons, Joseph loses his hand, which removes the only tool that - with proper training - could have opposed the threat.
The demons ravage the world, conquering both nations. And Joseph, and his ragtag team must oppose the foe against which apparently there's no power that could oppose them. Still, a new plan is forged, involving restoring Joseph's lost hand, and facing the enemies after mastering his powers.
Here we have two considerably distinct situations placed apart by a plot twist. If you pay close attention to minor clues and discover that Jago is indeed acting on malevolent motives, the twist becomes a part of the first situation, a fool's run towards inevitable doom. Still, most players will miss the hints, and the twist comes as a total surprise.
Now, these were situation vs twist. Let us add complication to the set.
The complication is the enemy army. It exists as a side thread that adds to the complexity. At one point Joseph faces the king. The king realizes severity of the situation but can't offer Joseph any major help. The quote would go along the lines "If it were a different day, I'd offer a unit of finest soldiers to your case. But with war looming over our borders, I can only spare this one soldier" [who joins your ragtag team]. The situation is the threat of demons and need to find rings. The complication is the looming war. It's not the central plot point, but it affects a lot. Prices and supply change rapidly. Trust is in short supply as enemy spies roam the realm. It creates minor twists, as villages that would welcome you peacefully, conquered, become a battlefield. It changes the game, but in a manner you can expect. A complication is a mutation of a situation, something that makes it less straightforward, requires additional measures, add difficulties. In your example, let us imagine Ralph suffers crippling memories of his prior, lost love, that keep him from acting rationally. Or he lives under repressive totalitarian government, that makes access to medicine difficult, and forces actions of dubious legality in his hopeless struggle. These would be complications. Ralph's wife's request for writing the will is not central to the plot. It affects Ralph, but it will not change the outcome. It's a point to deal with, but it's the sickness that's central to the plot; the request may be heart-wrenching, but it is meaningless in the long run.
- Situation: Central to the plot, constructed, long-run, the outcome can depend on it.
- Twist: Unexpected change, affects the situation, may change the outcome or provide an unexpected complication (or resolution).
- Complication: Not central to the plot, constructed, more or less long-run.
Of course a complication may turn into a plot twist (Ralph's wife was infected by government agents because she "knew too much", the story becomes a story of revenge), a plot twist may turn a situation into complication and vice versa, a complication may become a plot twist (suddenly the oppressive government finds Ralph's wife's case to be a viable PR stunt, giving her medication not normally available to common citizens), and so on. As stated before, the borders may be blurred, and it's merely an attempt to qualify elements of prose, something that isn't easily qualified...