@Lucia: You need to have low Subtle (basically get caught). Specifics:
On the Block: Murder the Consort (best with death magic, poison has the risk of you dying). With a Subtle below 50, the Monarch uncovers what you did and beheads you.
If You Stab at the King, You Must Not Miss: Try and fail to kill the Monarch with poison–you’ll end up dying yourself. Again, I believe this requires a Subtle below 50.
Your Sibling’s Keeper: Okay, I was wrong. “A Death in the Family” is the one where your sibling tries to kill you. THIS achievement you get by killing Don Felix, Rosa, and Mateo/Magdalena.
I also did a bit more scoping around:
Double Triple Cross: Unlocked when Ramirez betrays you with the life mage potion. Basically, fight him, threaten to use the death magic curse, and decide to kill him anyway. He’ll leave behind a defective potion and Antonio will be born a death mage.
Honeytrap: Use sex to control de Vega.
The Heir Presumptive: Put your child on the throne despite a weak claim. Best way to do this is to not marry the Monarch, NOT have Antonio be a Life Mage, and execute Juanita. Then defeat Tomas and make it to the end with the Monarch. They’ll die, and since there’s no one else in the country with royal blood, the throne will pass to Ricardo.
Live and Let Die: Survive the Monarch’s paranoid by throwing your ally/lover (de Vega/de Mendosa) under the bus. If you have a good enough relationship score with the Monarch, they’ll believe you and kill de Vega/de Mendosa, leading to the “Til Death Do Us Part Ending”. This is by far the easiest way to get that ending.