They sound almost alike, but they are not the same.
The usual mistake of using one form by another, so it should be clear what is each one:
Sino it’s an adversative conjunction that is written in one word and is used to deny something and then explain what it really is:
Antonio no es español sino colombiano / Antonio is not Spanish but Colombian
El azúcar no es amargo sino dulce / Sugar is not bitter but sweet
Si no is the conditional conjunction si and the negation no, and is written in two words.
Si no estudias no aprenderás la lección / If you do not study will not learn the lesson
ISi no llueve habrá sequía / f it does not rain there will be drought
To avoid mistakes just see if we can co introducing a word between the yes and no:
Si este verano no llueve habrá sequía. / If it does not rain this summer there will be drought
In the expression si no, whether the conditional conjunction si is always written without accent.