It can be normal. It is not unthinkable that the parents have asked them to live together so they can keep an eye on each other (IE The brother can keep an eye on the sister 
).
Heated arguments? Live with a Taiwanese for some time and you’ll get to experience some of those. Trust me, I’m happily married to one, but she argues more loud than I do. They can react … interesting from time to time, but they usually calm down rather quickly. I would let them sort it out themselves. If I liked to live with them, I would stay, if not I would leave. The knife thing should be discounted.
