NTA.

When my mum lost her hair, I remember seeing how painful it was for her.

You have cancer, you shaved your hair because of how painful it was. It isn't your job to cater to the whims of a cousin who can't be bothered to empathise with your situation.

She's TA for only caring about your hair and her wedding.

I'd be shocked if anyone isn't appalled that your cousin was able to carry on with the bridezilla behaviour to a 13 year old who is going through chemo.

I saw what my mum experienced, and can honestly say I don't think I would be able to go through that. You're only 13, and I can't imagine how you're dealing with the chemo and being nice to your cousin.

Your cousin is one or those people who will only use people to get what she wants. She's proven it by what she's just done here, please pay attention to this.