I read about demonstratives in a kreyòl grammar site. Is it correct for these meanings? sa a(this), sila a(that), sa laba a (that over there) and plural sa yo(these), sila yo(those), sa laba yo(those over there) Are there emphasis for them?

They are pretty much correct, but let me just add the followings:

sa or sa a will translate both this and that in English.
example:
nonm sa or nonm sa a → this man / that man

sila or sila a will translate both this and that in English.
example:
bagay sila or bagay sila a → this thing / that thing

sa yo or sa a yo will translate both these and those in English
example:
pawòl sa yo or pawòl sa a yo → these words / those words

sila yo or sila a yo will translate both these and those in English.
example:
moun sila yo or moun sila a yo → these people / those people

Here are some examples of the usage of  sa and sila in Haitian Creole.
1. Nonm sila a se yon pwofèt. or
    Nonm sa a se yon pwofèt.
    This man is a prophet.
    both Creole translations with sa or sila work just fine.
    some people say "sa", some say "sa a", and some say "sila"
    I remember when I was little, I did notice the bible used a lot of "SILA". ....So I do associate SILA with prophetic talks, poetry, lyrics ... and also older people from the outskirts of the country said "SILA" a lot too.  But it does not have to be confined to that. 

2. Ki moun ki pwopriyetè kay sila? or
    Ki moun ki pwopriyetè kay sa a?
    Who owns that house?

3. Bagay sila yo, lè nou fè yo, lage nou nan tèt chaje.
    Bagay sa yo, lè nou fè yo, lage nou nan tèt chaje.
    These things, when we do them, get us in trouble.

4. Ou wè zile sila yo?  Se moun rich sèlman ki abite la.
    Ou wè zile sa yo?  Se moun rich sèlman ki abite la.
     You see those islands?  Only wealthy people live there.

Sila can be used as demonstrative pronoun → the one.
5. Sila ki va genyen batay la se sila ki pèsevere a.
    The one who will win the fight is the one who perseveres.

6. Ankenn moun pa gen dwa al nan chanm sa a sof sila ki gen kle a.
    No one is allowed in this room except the one who has the key.

7. Sila ki mèt kannari a, se li ki mèt dlo a tou.

      He who owns the water jug also owns the water. (Haitian Creole proverb)

      The one who owns the water jug also owns the water.

Sila or sa can be used as demonstrative pronouns: this one, that one

8. Konbyen sila yo koute?
    Konbyen sa yo koute?
    Konbyen sa a yo koute?
    How much do these ones cost?
    Again, here, some people say "SA YO" and some say "SA A YO" depending on the region they come from.

9. Mwen pa vle sila a.  Li twò piti.
    Mwen pa vle sa a. Li twò piti.
    I don't want this one.  It's too small.

As far as sa laba a or sa yo laba a is concerned, let's see some examples and use them in sentences:
Basically:
isi translates here
la translates there
isi a, isi translates over here
laba a, laba translates over there

10.  Ou wè madanm sa laba a? Se manman m li ye.
       You see that woman over there?  She's my mom.

11.  Al pran chèz sa yo laba a pou mwen. Go get me these chairs over there.
       or
       Al pran chèz laba yo pou mwen. Go get me the chairs over there
        or
       Al pran chèz ki laba yo pou mwen. Go get me the chairs that are over there.

12. Moun sa yo laba a se fanmi m. These people over there are my family.
      or
      Moun laba yo se fanmi m. The people from over there are my family.
      or
      Moun ki laba yo se fanmi m. The people that are over there are my family.

Haitian Creole ↔ English Reference, Look up Haitian Creole and English Words