My mother in law

Kassie • I'm Kass, me and my husband are expecting out firstborn son in May 2016. We love our little Nash Reagan Garcia 😘
 Apparently my mother in law decided she will be called "mom" by my son on the way.. Please help me find a respectful way to tell her that I'm not comfortable with that.. Maybe some suggestions for other grandma names would help? What do I do?!!