My older sister bites her boyfriend in arguments is this behavior normal?

Last night my sister's boyfriend who happens to be living together called us that my sister and him got into an argument out of nowhere my sister bites him. Is my sister dealing with mental issues or having a mental breakdown? This behavior is not normal. Ok yes woman we are very emotional but biting your partner is not normal. Last time I allowed my sister to stay over she scream F*** Zonic at the top of her lungs and punch my walls and made these huge wholes on my Walls which as very disrespectful of her. Should we be worried?

My older sister bites her boyfriend in arguments is this behavior normal?
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