Aunt and uncle is strange

Isn't it strange that your parents sister or brother is seen the same as your parents sister or brothers husband or wife??
And for the reciprocal niece and nephew. Your brother or sister's children is seen the same as your husband or wife's brother or sister's children.

Also with brother or sister in law, your husband or wife's brother or sister is seen the same as your brother or sister's husband or wife or I believe even your husband or wife's brother or sister's husband or wife