What should i do when my little cousin is being rude and behaving terribly?
Just be the more mature person and be firm when letting your cousin know her behavior is not cool. Sometimes kids do this to demand attention. If it gets out of her hand, talk to an older person at home about it.
Ignore hm. He will get the hint if you don't respond to bad behaviour.
when he says or does anything wrong, tell him to quit as soon as you can, or he'll keep doing it. and when you say something about quitting, say it in a strong voice to let him know your not joking.