My [17M] big brother [20M] wants me to write a love letter to his girlfriend [20F] for him, but I think he should do it himself.

My big brother has been dating his girlfriend for almost two years now. For their two year anniversary, he wants to give her something special. He asked me if I could write a love letter to her for him. I do a lot of writing in my spare time, and I write a lot of stories on other Forum Sections (obviously not on this account). I've also always gotten straight A's in English, and I consider myself a pretty strong writer. My brother on the other hand always got C's in English and struggled with writing assignments, so it makes sense why he's asking me to write to his girlfriend.
But I feel like that defeats the whole purpose if I'm the one that writes it. I don't know much about my brother's girlfriend. I've talked to her twice, she's pretty and seems like a nice woman, but that's all I know about her. I could write a really nice lovey-dovey-fluffy letter, but it would be my words, and it wouldn't be genuine.
I told my brother that he should write it himself, but he says he doesn't know how to start. He can't put into words his love for his girlfriend and says I'd do a better job at it - even though I don't know anything about her. My brother has been begging me for the past few days to write it because their anniversary is coming up (April 2nd). He said it would make her really happy because she's never received a letter before, and he'd owe me big time for it.
I can tell my brother loves his girlfriend very much. He talks about her all the time when he comes home from school to the point where I need to tell him to stop because I'm not interested. My brother shows a lot of affection for her when they're together, and I find it quite endearing. I don't think it would be that hard for my brother to write a letter because clearly, he's deeply in love with her.
How can I get my brother to understand that it would be a better if he was the one that wrote it for her?
TL;DR: My brother has been begging me for the past few days to write a love letter to his girlfriend for him, because I'm a strong writer, and he doesn't know what to say or how to start it off.


Source.