Agreed, I would keep the original piece and work around it. If you want the whole sleeve to be a tribute to your brother how about incorporating something that he liked a lot or something unique about him? As far as the religious side goes you have many options as ontwo pointed out, I guess it comes down to personal preference what you want included.

Have you thought about colours at all? If you stay in b/w Id really vote for keeping the one you have, I think its a nice piece although youre right, its not too large in itself.

that looks pretty good man, I like the text and the rays would def look cool with some good shading and blending with the text.

I don’t have the photoshop skills that you guys have but….

Those hands, with the rosaries.. what if you added an angel in profile looking up towards your should wrapping around the back of your arm to the hands? I mean, the hands that are there will be the angels hands and you just add the body to it with wings and a robe and all that jazz. Then you could add a sun and rays of light and clouds that he is looking up towards. I could be a nice reminder of your brother and a real piece of art at the same time.