I don’t think you should write her a letter at all. She obviously wants to be left alone in peace. It may be the repetition that’s getting to her. She might even be musical. Or she might be tone-deaf and it’s just noise. I would definitely close the windows and drapes. An air conditioner will let out more sound when it’s not on. But by all means get a bigger harp. Is it pointing toward the window? Adding more carpet, drapes, tapestries, will all absorb the soundwaves, and force you to play with good tone for the lack of reverberation. People are very different at different ages and have different needs and concerns. If she will always live there, and you will be leaving for college soon, then be patient. But not mute. Adapt as you can acoustically and forget the rest. Or can you move the harp to the other side of the house? I suppose it doesn’t get hot enough to really use air conditioning much. I have a small fan I put at the front of the harp base on the floor, so it blows up the sounding board. That keeps me cool when the room is warm. Good luck. It can certainly be better or worse.