People keep throwing the term tsundere around regarding the cousin, but she isn't like that at all. Her coldness towards the MC is a key trait for an ice queen, which makes her a kuudere, not a tsundere.
Tsunderes are sociable and far more expressive with emotions, here we see Mashiro's emotions are pretty controlled with only a cold glare (kuuru side) and hidden blushing (dere side). She's kind of a loner too and not very sociable.