If the cousins are MALE then yes…Coz you never know when shetan put dirty thoughts in your heart ( sorry if it’s off topic ) but It’s true… And If cousin is female then may be mom is concern that she her daughter will ger Asar from her cousin..
Now with sibbling This answer Is from My Nano… She says you dont go to any traning school But Your home is your traning school…So how you behave with your sibbling will teach you or you will learn from this that how to behave when you are with friends or cousins… I used to fight with my sister for like why did u use my lipstick… Or we used to prank with each other…My mom never said that DONT do this.. But she used to say things should be In limits.. Like If I was fighting with my sister my says No TAna or Tanz No bad language…
And for casually talking I dont see any thing wrong with this 