AIBU to feel hurt by my partner's sibling's comment about our baby?

To be honest, your question is odd, I have never heard anyone ask what the baby’s name meant (as one of the first questions anyway), she probably thought that you were digging towards the uniqueness maybe? Her response is out of order of course, but you questioned your hormones in your initial post, but shes also very fresh on the PP train.

Not sure what the gender disappoint has to do with this? But I doubt that they wouldn’t change their baby for the world now.

honestly, you sound like you have a great relationship, maybe let it go. Or check in with her and raise it just the two of you. She could very well apologise.

This is the kind of thing one of my brother in laws would say…. Unfortunately it’s his attempt at humour when actually he just comes across as a dick. Just either completely ignore it or act like ur confused n hopefully having to explain why it’s funny will let him know he’s not funny

Remember the saying "you can't choose your family ".
Two of my now grown up kids had some cousins same age, when they were small all was well, growing up not really interested in each other, very different parenting and financial backgrounds. My kids were more academic and down to earth, cousins latest expensive brands and tech very image conscious and brought up in a city.
All kids lovely but different interests and personalities.
The competition is only that if you let it. Ignore any unpleasant comments and focus on your new family. 💛

They sound a bit dickish but I’d just let it slide this time around, not something worth being upset about.

Could be that it’s not a very nice name and they’ve had comments before so are a bit on the defensive.

His brother also asked why we chose “Charlie” when he came to meet our baby so it wasn’t seen as a big deal or unusual conversation.

It's not terribly deep but it does sound a bit like he can dish it out but not take it. You'd said it was a nice name, your DH had asked where it was from. Assuming you asked the question in an interested way (which I'm sure you did) it seems a logical place for the conversation to go to me. And even if he for some unknown reason felt you were being too intrusive he only had to reply non-commitally. There was no need to leap to the 'better than yours' comment.

I'd talk to DH and get his take on it before you make any decisions. What you don't want is your child being constantly put down to make themselves feel better about you getting the sex they wanted and they didn't.

Pardon us for showing interest!

The way I read this it was unspeakably rude, whether it referred to the name or to your baby. Your partner sounds as if he was at fault for not calling her out.
I’d avoid this person in future, but if you have to see her again the best response to a comment like that is to look them in the eye and politely/calmly ask “What did you mean by that?” then equally politely say “I see” in response to whatever they say and walk off.

No it isn’t. It’s a normal way to show interest in a name.

You should have immediately said, are you seriously having a competition between babies? You and your husband should ask for an apology, or an explanation of this weirdness.
If they did think you were insulting the name, you can explain you really weren't, but their reaction is still weird.

So everyone is tired...drop it.

If it's an unusual name she's probably sick of being asked what it means. Not all names have to have meaning. Ive no idea what either of my kids names mean.

Thats horrible. They sound like a dickhead.