The girl's clothes are more expensive than the boys.
The camel is called as the ship of the dessert.
My sister said these things; are they correct? I'm wondering if the grammar is correct here.
The girl's clothes are more expensive than the boys.
The main problem I have with this sentence is the apostrophe mismatch. The apostrophe indicates possessive (in this case, a plural possessive), so you should write:
The girls' clothes are more expensive than the boys'.
However, if your sister said these to you aloud, there's no way to tell where she put the apostrophes! So the error is not in what she said, but in what you transcribed.
As for where the apostrophes go, you can find out more about that here.
The camel is called as the ship of the dessert.
One problem here: the extra as. However, either of these are correct:
The camel is called “the ship of the desert.”
The camel is known as “the ship of the desert.”
I've added quotation marks for clarity, but they wouldn't be required. The major issue is the way the verb phrases is called and is known work; only the latter takes the "as."
girls' ... boys'
is correct if, e.g. you assume that you are comparing the prices of clothes in the girls and boys sections of a store, but that's not the only valid way to interpret that statement (without any more context). girl's ... boy's
would be correct if you're comparing a particular girl's clothes to a particular boy's clothes. girl's ... boys
(the original) would only be valid if you were comparing a particular girl's clothes to the price of boys (e.g. you're buying boys as slaves...but let's hope that's not the case :P).
I'm afraid they are not. When you use the definite article the, it means that particular person/thing. So, in your first sentence, you are talking about that one particular girl or one particular shop's clothes. In such case, zero article (no article) can be used. Remember, if you are talking about the mass in general, the can be omitted.
Girls' clothes are more expensive than boys' (note the apostrophe as well - you are talking about the girls' clothes and not a girl's clothes).
Camels are called 'the ship of the desert' (note 'camels, all camels as a mass' and also, it's desert and not a sweet dish -dessert!).