"Sí" is also the reflexive form of the third person pronouns (él, ella, ellos, ellas), which must always be preceded by a preposition. This "sí" is completely unrelated (has different etymology) to the word "sí" meaning "yes".
Reflexive means that the subject applies an action to themselves. A possible translation in some cases would be "preposition+himself/herself/themselves". The word "mismo/misma" is often appended to emphasize the reflexiveness of the action, but it is not mandatory. For example:
Dijo para sí (mismo)... / He said to himself...
Lo alejó de sí (misma) / She pushed him away from her(self)
El frío en sí no me molesta: me molesta no poder ir a la piscina (The cold weather itself does not bother me: what bothers me is not being able to go to the pool)
The construction "entre sí" means therefore "among themselves", which might more naturally be translated as "each other" or "together" in some cases:
- Los planetas se atraen entre sí (The planets attract each other)