The Lannister name doesn't mean what it once did.
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.