Martiens IN German

I am looking to find out who my grandfather was. I have recently found out that he may have been german. My mothers maiden name is martin but it is spelt differently in german with a larger s in front of the small s. He had about 10 children.
One of mums brother's is called Lou, now deceased, and had sons Carl, Denise and another, not sure of name, can find out. His wife is Pam, just recently deceased and mum is the last martin alive. I think she knows but not willing to tell. Mum married dad, Ian Roxburgh who also is deceased. We all live in New Zealand so anyone with info out there would be appreciated. Thanks Jan