The Proto-Indo-European root *se- is generally taken to have been a reflexive pronominal root meaning "self", i.e. indicating that a word refers to the same thing as some other word (De Vaan 2008, Philippa e.a. 2003-2009). The root *so- is a different pronominal/demonstrative root that was so far as I know not reflexive. So I would not trust Wiktionary here.
The Latin reflexive pronoun se is from Proto-Indo-European *se- (De Vaan). I wouldn't trust that page from the University of Texas too much: their "own, possess" seems speculative. Perhaps it was based on Pokorny, but I wouldn't depend on it either way.
According to De Vaan, Latin si and sic are from Proto-Indo-European *so-, as expected. I don't think there is any doubt about this. It is related to English so and many other reflexes.