Don't forget she's an airbender so under Tenzin's command.
She has no problem kissing MC in front of her dad, who is clearly not happy about it yet she did it, nor did she tried to hide her feelings afterwards. She did, however, obeyed Tenzin for not leaving her room at night, so - it's not that clear cut. I get the impression that though she's over 18, since she has to be around Tenzin (because airbender) and live in his house, she can not do at his home as she likes ("my home - my rules!"). iirc she does feel suffocated there and said something like this when she got to the pole - that it's sort of a vacation for her.