MIL/baby name issues. Am I overreacting?

FTM here. I'll keep it short because I'm sure many will understand how MIL stories could never end. My husband is Tibetan and I'm Caucasian. His family has a lot of pride in their culture, which I support. In picking a name for our son, we've chose Marcus Tsogyel, the latter being my husband's proper Tibetan first name. Well, my MIL insists on calling the baby Tsogyel and is getting upset at me for correcting her. She does the same to his sister who's 5 year old daughter has a non-Tibetan first name and Tibetan middle name. (At this point the sister has stopped even trying to correct her or say anything.). Am I overreacting in getting upset/being bothered by this?