In Alvin and the Chipmunks: The Squ

In Alvin and the Chipmunks: The Squeakquel, after Brittany asked Ian if he could represent them, Eleanor says, "And hang out with the Chipmunks!!" Both she and Jeanette giggle excitedly, meaning they are already smitten with The Chipmunks from fame and inspiring them to sing.
In Alvin and the Chipmunks: The Squeakquel, after Jeanette and Eleanor talk about Simon and Theodore, she says to them, "Yeah, I know. But, guys, remember what Ian said? We can't trust them." hinting she has a crush on Alvin, but has no choice except to follow Ian's rules.
In Alvin and the Chipmunks: The Squeakquel, Ian lies about Alvin, and Brittany becomes really disappointed and appalled, showing she was interested in only meeting Alvin.
In Alvin and the Chipmunks: The Squeakquel, Brittany can be seen sitting right behind Alvin on the motorcycle holding onto him tightly.
In Alvin and the Chipmunks: The Squeakquel, near the end of the movie, Brittany and Alvin dance together and then smile at each other happily.
In Alvin and the Chipmunks: Chipwrecked, Brittany tries to comfort Alvin after he doubts Dave will rescue them after he has driven him insane for a few years.
In Alvin and the Chipmunks: Chipwrecked, when Brittany tries to comfort Alvin, she has a pink mark on her cheeks which signifies that she is blushing.
In Alvin and the Chipmunks: Chipwrecked, Brittany paused and smiled at Alvin when he called her smart.
In Alvin and the Chipmunks: Chipwrecked, during "Born This Way/Ain't No Stoppin' Us Now/Firework," she and Alvin are seen frequently dancing and signing together and in the end of the song are seen waving to the crowd with an arm around each other.