Did an elder from your family tell you that it was bad to be casually talking with your siblings/cousins??? Or is that a doubt/assumption that you have based on fear?
There shouldn’t be a problem with you engaging in banter with siblings and cousins unless:
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You’re super loud…and are disrupting the function in some way. And maybe you’re drawing negative attention toward yourself.
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The content of your bantering is seen as inappropriate by the elders. Perhaps you and those among your age group might think there’s nothing wrong with your conversation…but maybe the adults think otherwise. What if they think your friendliness is “flirtatious.” Keep in mind that there is a GENERATION GAP between you and the aunties/uncles. Due to this generation gap, there will be differences in the perception about what is considered “appropriate” or "casual.
Also, as Kinzz mentioned earlier…in some families, interacting closely with a cousins of the opposite gender may be frowned upon…although your own heart/intentions may be clean.