Why does 'kau' feel so aggressive when I use it, but my Malaysian friends use it constantly?

I’m a heritage learner trying to connect with my cousins in KL, but every time I try to use 'kau' or 'mu' to sound more natural, they laugh and tell me it sounds like I’m looking for a fight. I understand the informal context, but I’m struggling to bridge the gap between textbook Malay and the reality of sibling banter. How do I gauge the right level of intimacy without sounding like a rude foreigner?