AITA for refusing to sing at my brother's wedding?

My (28M) brother (33M) will be getting married in the fall. The two of us are fairly close, more so in the past few years, which is to say that he is not ignorant about past events in my life.

Recently, I got invited to dinner by him and his fiancée. The two were very obviously buttering me up to ask me something the whole time before my brother finally told me:

"So, [fiancée] and I have been talking, and we'd really love it if you sang our first dance song, just the one song. I know you don't really sing anymore, but I dug up some old videos of you singing and she loves your voice just as much as I do. This could be your wedding present to us!"

For background: I used to sing all the time. I formed a band with a bunch of my friends in high school, and we were very minorly successful. We had a YouTube channel with a few hundred subscribers, and there were a handful of people that had their own recordings that were posted to their own pages.

In this band was my best friend since second grade, Mason (not real name). Unfortunately, when we were 17, Mason took his own life. Additionally, I was the one who found him.

The band dissolved almost immediately, and our YouTube channel and all our own videos were taken down. Since then, I have NEVER sang. Singing without Mason felt wrong, so I didn't. Not in the shower, no karaoke sessions, not at church, never.

My brother knew all of this, but I wasn't sure if his fiancée did, so I started off with "Sorry, I'll have to refuse. You know I don't sing anymore" in the interest of not totally ruining dinner.

My brother was annoyed as hell at this. "Come on, it's for my wedding, it's just one song, I'm not asking for much here" and so forth.

I continued to politely refuse and left soon after. Shortly after, I start receiving countless calls from my mom, who also knows the reason why and decided to harass me about not singing. "It's your brother's wedding! I think you really should see a therapist about this, we all love your singing voice and it's been 10 years since any of us got to hear it!"

The two have since decided on a new tactic by saying that my song will be a good way to honor Mason's memory.

The point about therapy aside (I've been to lots of it. I'm at peace with my decision to stop singing) AITA for refusing? It's clearly important to them.

EDIT: To clarify, I have really only ever performed with Mason. The joy I got from performing was not the act of singing itself, but from performing with my best friend.