The words saner, seiner sound the same but have different meanings and spellings. Why do saner, seiner sound the same even though they are completely different words?
The answer is simple: saner, seiner are homophones of the English language.
comparative form of sane: more sane
a fisherman who uses a seine to catch fish
a fishing vessel used for seining
Definitions from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").