Writing the letters – the stupid, foolhardy, insane letters – is harder than it has any right to be. He's sure he had penmanship lessons for years as a child, certain he should know how to do this, but the quill doesn't obey his hands and even when it does his hand shakes. The end result is awful but he doesn't feel comfortable borrowing even more of Nem's parchment and ink to try for perfect, nor does he know how he'd dispose of the scraps.
He writes to his family – what's left of it – and isn't sure if he doesn't want for Nem to investigate further, perhaps to find him out. He doesn't like this, living a double life. He wasn't made to spy no matter what everyone else thinks.
That thought surprises him in his bitterness. Even in Azkaban he hadn't bothered with feeling bitter about that betrayal, even when he was wondering how easy it had been for them, but it was different now with the things Nem had told him. Oh, they were a child and Andromeda had made sure he'd never been near them when he was in a dangerous mood, but they'd still held onto that belief all through school and all through auror training. How strong his poor cousin had had to be on his behalf.
The least he could do is write. (And yet, he doesn't tell them about Pettigrew.)