Should people call their parents-in-law "Mom" and "Dad"?

My brother in law started calling my parents "mom" and "dad" -- and in turn my sister now calls his parents "mom" and "dad".

This seriously offends me. My parents earned the title of mom and dad. They RAISED us --they cared for us, educated us, taught us values, loved us, and corrected us when we were wrong. My relationship with my parents is one of the most important relationships of my life and one I don’t take lightly. I would never think to call anyone else “mom” and “dad” because no one else has even remotely earned it. It would only cheapen the relationship for me.

I think my brother in law is being too familiar with my parents, and disrespectful to his own (and vice versa for my sister). Perhaps it's because I don't feel he is part of our family, or that I don't WANT him to be a part of the family, or that I'm a bit jealous...

You see, my sister (1-1/2 years older than me) has a mean and manipulative streak about her -- one which I have had to deal with my entire life. About 10 years ago, she found religion and now she is mean, manipulative, AND pious. About 3 years ago she married this fellow insanely religious blowhard who seemed to imitate her behavior towards me and my son. So I may be predisposed to see the worst and be irritated about it. And about 1 year ago my sister and brother in law moved within a mile of my parents home (I live about 45 minutes away from them). So maybe I'm a little jealous and this is just aggravating it.

Are my feelings valid? And what can I do about them if they are not?