Mom refuses to change baby's name for in-laws, 'his ex had a son with that name.' AITA?

"AITA for refusing to change my baby's name?"

My partner (M34) and I (F32) recently welcomed our son Liam Alexander into the world. My partner's sister (Katie) has 2 kids (James 6 and William 4) with her husband (Mike), and he has a daughter (10) with his ex.

My partner and I didn't settle on a name for our son until the day of his birth. I have always loved the name Liam, for its sound and meaning, and was over the moon when my partner finally agreed to one of the names on my list. 24 hours after Liam was born our families came to meet him, his sister being one of the first.

We announced his name to her and her immediate response was "Oh, I've always loved the name Liam, but Mike wouldn't let me use it for our first son! Could you change it? Maybe call him Alexander as his first name? It sounds nicer!" I firmly said I loved the name too, and while I empathize with her, we would not be changing his name, or the order of his names.

I thought that was the end of it until she visited us at home, a week later, and brought it up again. The reason Mike didn't let her use the name is because his EX has a son with that name. I told that shouldn't have stopped her from using the name she loved.

Those boys do not know each other, do not go to the same school, don't have the same last name and basically live separate existences outside of their shared half-sister. I also said she could still use the name in future, it's common in my culture and tends to serve as a bond between the kids.

For clarification, Katie and I have never discussed baby names, and we have no name pacts. The only reason I feel like TA is because Mike and his EX have a toxic coparenting relationship, and this understandably affects Katie. Katie and I have a good relationship but she doesn't talk about Mike's ex, so this is as much as I know. AITA?

Here's what top commenters had to say about this one:

Tricky-Jellyfish-341 said:

NTA and hold your ground. Sil has a lot of irrelevant (to you) drama and history surrounding the name. You don't. Why in the world would you try to make your sil feel better about this? You named your baby your favorite name. Nothing else actually matters

SavingsRhubarb8746 said:

NTA. It's remarkably rude to ask someone to change their child's name, and even if it wasn't, you have made some perfectly good points about the fact that Katie can, if she wants and her partner agrees, name a future child Liam.

Nobody_asked_me1990 said:

NTA. That was not a reasonable request in the slightest, and it was rude to even ask. No prior agreement, no communication, nothing that would be a reason to avoid a name.

ilikedrawingandstuff said:

NTA. I always marvel at the audacity to ask someone to change an already given name...

Chase-Rabbits said:

NTA. Sounds like her and Mike have some problems that are not your job to resolve.

sherlocked27 said:

NTA. It’s literally nothing to do with you. She should take it up with her husband not you.