In S03E08 of Game of Thrones, Second Sons, Cersei Lannister and Margaery Tyrell are walking around the Sept of Baelor discussing their upcoming marriages to Loras Tyrell and Joffrey respectively, when this exchange takes place:
Margaery: We're going to be sisters soon, we should be friends
[Cersei explains the meaning of the song "The Rains of Castamere" at length]
Cersei: If you ever call me sister again, I'll have you strangled in your sleep.
Which really sets the tone for their relationship. But then in S04E05, First of His Name, after Joffrey's death and Tommen's ascension.
Margaery: We may be faced with an alarming number of weddings soon.
[Cersei laughs politely]
Margaery: I won't even know what to call you; Sister or mother
[Cersei looks into the distance inscrutably]
Which makes me wonder if I'm missing something, the Tyrells must know what a reputation she (and most of her family) has for violence, so why did Margaery risk it? Could it have been a sign of dominance? It didn't work out well for anyone else who tried to get a rise out of her (Eddard, Petyr, Tyrion).