I've noticed that in most brumira fanfics/headcanons, Mirabel is portrayed as unapologetic and fearless regarding her feelings for Bruno. This doesn't really seem right to me. Throughout the movie, we see Mira as a mature girl, loving her family too much and in constant fear of letting them down. I think she would definitely experience heartache and shame in processing having romantic feelings for her uncle. I can see them both in denial for years, until finally some kind of breaking point comes
Ah, Mirabel-oriented angst. You raise a very interesting point. Of course, everyone is welcome to their own headcanons and interpretations of characters, but I have to agree with you. When we look at Mirabel's arch in the film, there is a lot of basis for angst surrounding her attraction to Bruno when she does recognize it. I've focused a lot on Bruno's POV, but I do think they both would experience shame, guilt, denial, and attempt to repress their feelings for some time. Though, I think the reasons for this and how they manifest slightly differ based on their experiences with the family and what messages they have internalized.
Consider Mirabel. As you pointed out, in the film, we see Mirabel constantly trying to serve the family, uplift them, and make them proud, but we also see throughout the film that no matter her efforts, she somehow lets them down. Everyone in the family is working hard to prove their worth to Alma, but I think Mirabel probably works the hardest because she never got a gift. And in a family where how useful your gift is determines your value, Mirabel was given the message that she is essentially worthless since the age of 5.
Then we have Bruno. Bruno has a gift, and at first it was incredibly valued for its power. But that power quickly became feared because many people really only want to see the future they want. Not of what will actually be. And as Bruno's gift became notorious, so did he. By association, people believed Bruno made bad things happen. It seems that even his presence could be considered a bad omen, let alone anything he said, whether it was related to a vision or not. It's unclear how quickly it fell apart for Bruno, but it is clear that the message Bruno received is that he is inherently bad.
And they both internalized those messages. It is clear in the film. Mirabel must save the miracle to contribute the family because she is worthless. Bruno hides in the walls because he is essentially trying to contain the "bad things" that happen around him from his family.
To illustrate it, I think the internalization goes something like this for each of them:
Mirabel: I did not get a gift, I get in the way, I mess everything up, I can't do anything right = I am worthless.
Bruno: My gift upsets people, it makes bad things happen, I make bad things happen = I am inherently bad.
So how would these messages manifest when Mirabel and Bruno develop feelings for the other?
For Mirabel, I think her attraction and feelings to Bruno would tap into the fears of, "I can't even be a functioning member of the family without doing something wrong (developing feels for Bruno).” And lead into thoughts like, “What is wrong with me? This needs to stop because it could destroy the family and destroy my relationship with Bruno."
For Bruno, I think it would tap into the fears of, "I am evil." More specifically because he would have developed feelings after Mirabel saved the miracle, I think his thoughts would play out like, "I have evil thoughts for the one person who was willing to save me, who I should not be feeling for like this. Look how evil I am. I should have stayed away."
I do think that if someone was going to be unapologetic and brave about their feelings, it would be Mirabel, but I also think that she wouldn't start out at that place. I'm happy to elaborate on this, but I think I'm getting away from the original ask. Ultimately, I do agree; there is a lot of shame, denial, and angst to be had on either side of this pairing, if you are so inclined to explore it.
