please help

Question

in these two examples:C'est le fils de Martha.He's Martha's son.Qui est Sylvie ? - C'est ma sœur.Who's Sylvie? - She's my sister.

surly if i say: “elle est ma sœur” or “il est le fils de Martha” that is a correct sentences right??????????? i m sure/ think i ve heard that in french!!!! why shouldn’t it work????

this is very confusing

Accepted Answer

It sounds like you are still thinking in English and want the French language to follow the same rules. It doesn't. 
As the lesson notes (almost always) when you want to say "it is (article) noun" you need to use « c'est » not "il/elle est". Why ? - because that is the way French grammar works. 
I have never heard a native French speaker do otherwise - why would they when they learn the 'rule' from a very young age with constant parental (and teacher) correction ! 
Have a read of a couple of other articles on the same subject - you will see that there are occasional exceptions, but essentially you will find that aside from these exceptions, you will not use « il/elle est un/une/le/la/other article NOUN ».
https://www.lawlessfrench.com/grammar/cest-vs-il-est/