confused about 'C'est' when taking about people?

Question

C'est ma sœur. and C'est le fils de Martha.

Can we not just use ''Elle est ma soeur'' and ''Il est le fils de Martha'' ?

Edit: Nevermind, I asked my French friend who told me that you specifically cannot say 'Il/elle est un/une/mon/ma etc'

This rule only applies to the pronouns 'Il' (he) and 'Elle' (she).

So I've answered my previous question, so No you cannot say ''Elle est ma soeur'' it has to be ''C'est ma soeur''

I felt like it wasn't explained very in the lesson! (sorry!) I hope anyone seeing this message finds this helpful.

Accepted Answer

Hi Darrell,
I think you have answered your own question but if you look in the Q&A section after this lesson you will see that it has been asked and answered along those lines many times before.
Bonne Continuation !

Answer

I don’t understand why this isn’t stressed more in the lesson, as it’s a confusing rule for English speakers and a common error.  The lesson does say "If it/he/she is is followed by un/une/le/la... (any form of article / determinant) - it is a beautiful dress / she is a nice person - then you will use c'est."  but the corollary isn’t emphasised - that you can’t say "il + être + un + noun" or "elle + être + la + noun"