A lot of people, even Filipino natural speakers themselves, get confused between silá and siná. Silá is a pronoun and shouldn't be followed by a noun. On the otherhand, sina is an article that can be used before a noun/s.
Si and Siná are articles used before name/s of a person/people.
Si is a singular article, while siná is the plural article, used before the names of two or more people.
Mga is used to indicate the plural form of a noun. Adding this before the noun shows plurality.
Notice in one example, plurality has been indicated by using the article mga in the phrase mga estudyante . Adding the article mga is optional since siná has indicated plurality beforehand.