Let Stannis and Roose battle. Let the enemies of the Throne slaughter each other and when they're done, seize Winterfell from whichever thief survives.

Littlefinger: Stannis Baratheon garrisions at Castle Black. He'll march south to King's Landing before the winter snows block his way. But first, he has to take Winterfell.
Sansa: You don't know that.
Littlefinger: I do. Once he liberates these lands from the Boltons, he'll rally your father's bannermen to his cause. With the north behind him, Stannis can finally take the Iron Throne.
Sansa: You think he'll defeat the Boltons?
Littlefinger: He has a larger army. He's the finest military commander in Westeros. A betting man, would put his money on Stannis. As it happens, I am a betting man.

Littlefinger: The Lannister name doesn't mean what it once did. Tywin is dead, he kept his house in power through sheer will. Without him, Jaime has one hand and no allies. Tommen is a soft boy, not a king to fear.
Roose: The Queen will be enraged.
Littlefinger: Queen Margaery adores Sansa. Cersei is Queen Mother, a title whose importance wanes with each passing day.

Brienne: Lady Sansa, before your mother's death I was her sworn sword. I gave my word I would find you and protect you. I will shield your back and keep your counsel... and give my life for yours if need be. I swear it by the old Gods and the new.
Littlefinger: Please, Lady Brienne no need for such formality. You were Catelyn Stark's sworn sword?
Brienne: I was.
Littlefinger: Strange I knew Catelyn from the time we were children, she never mentioned you.
Brienne: It was after Renly's murder.
Littlefinger: Ah yes.

Given the opportunity what do we do to those who've hurt the ones we love?

Money buys a man's silence for a time. A bolt in the heart buys it forever.

Baelish: Do you know what the realm is? It's the thousand blades of Aegon's enemies, a story we agree to tell each other over and over until we forget that it's a lie.
Varys: But what do we have left once we abandon the lie? Chaos, a gaping pit, waiting to swallow us all.
Baelish: Chaos isn't a pit, chaos is a ladder.

I did warn you not to trust me.

Ned: The king has no true born sons. Joffrey and Tommen are Jaime Lannister's bastards.
Baelish: So, when the King dies...
Ned: The throne passes to his brother, Lord Stannis.