my two daughter in laws and my son in law all call me by my first name and that's okay with me.
when i married as a 18 year old (crazy!), my inlaws never told me what to call them. i felt uncomfortable calling them by their first names but just could seem to call them mom and dad. for years i think i avoided calling them anything. finally (i was in the family 16 years) i started calling them by their first names.