Unfortunately, I'm not familiar with the Sinding.
However, I can offer my opinion of the order of difficulty (easiest to hardest) of the composers you mentioned. By this, I mean technical difficulty, not musical or interpretive.
1. Mendelssohn
2. Grieg/Sibelius (tie)
3. Faure
4. Rachmaninoff/Prokofiev (tie)
Keep in mind, this is only my opinion.