1. He often takes pictures of sunrises/sunrise.

(Do we have to use the singular form or the plural form?)

2. What's wrong with your brother?
- No, not at all.
(Is this answer suitable? Do you have other suitable answers?)

Yes, the plural is appropriate, assuming he takes pictures of sunrises on different days. If he's taking multiple pictures of a single day's sunrise, then the singular is appropriate.

#2: No, that is not suitable. It doesn't make sense. A suitable response would be, "There is nothing wrong with my brother." It could also be just, "Nothing," or "Nothing's wrong."