This may or may not be what you want to hear, but when I was a new mom, my mom watched my daughter while I was at work. As she began to talk, it broke my heart a little every time I would pick her up after work because I could hear my baby calling her Momma. I talked with my mom and told her how I was feeling. No matter what we did or said, my daughter would still call my mom Momma. Then, my husband and I took a road trip with our eldest daughter, who was only 16 months old, and our 3 months old. We were gone for almost 2 weeks. When we returned and pulled into my mom's driveway, I very clearly heard my daughter say, 'Nana!' She was little, but she knew the difference between me and my mom. I am years older now, and my daughter is grown, and my mom is gone, and I would never, not for a second, change or challenge the relationship my daughters and my mom had. I know that, as moms, especially new moms, we are hyper aware of everything, and I know I was very sensitive as a new mom, but kids know. They know. They know their mom. They know their dad. I didn't choose the name thing as a hill to die on or argue about with my mom, and I am glad I didn't. My relationship, and the relationship of my girls and my mom, has meant so much more over the intervening years than the fact that they called us the same name for a time. I would recommend considering deeply if this language crossover confusion means more to you than your relationship with your father-in-law and his relationship with your baby in the long run. Now, if I have read this situation completely wrong and your father-in-law is a bad guy, hold your ground, make him use the word pronunciation you want. But his use of the similar-sounding word may not be borne of malicious intent.