Some people use my mobile phone number to send malicious messages to relatives. Was my SIM card number stolen?

It is unlikely, because there is basically no case that the mobile phone number is stolen, and the mobile phone number basically belongs to a special card. Others use your mobile phone number to send messages to your relatives through software, which is not your number in essence. You can log on to the website of the number operation company to check your communication records. If not, that's the situation above. Of course, it is also possible that your mobile phone card has been copied by others, but this is rare. You should inform your relatives immediately and change the number.