She was tired after a long day working as a seamstress (sewer) in a department store and she felt she should have the right to sit anywhere she wanted.
Anywhere is a locative preposition. It has the meaning in any location. It is not a relative word like that, which or where or wherever. The original sentence could just finish with anywhere:
- She was tired after a long day working as a seamstress (sewer) in a department store and she felt she should have the right to sit anywhere.
The writer in the original example decided to use a relative clause to change the meaning of anywhere in the sentence. The original example doesn't use the relative word that:
- she should have the right to sit anywhere (that) she wanted.
Here we don't need to use the word that because there is a subject, she, in the relative clause. In this relative clause the word wanted is 'standing in' for missing words. We understand that wanted means "wanted to sit" in this sentence. If wanted wasn't standing in here, the sentence would read like this:
- she should have the right to sit anywhere (that) she wanted to sit.
There is a gap at the end of this sentence where the preposition is missing. We understand that it means:
- she should have the right to sit anywhere that she wanted to sit
The word anywhere is the antecedent for this relative clause. We understand that the clause is giving us more information about the word anywhere. Notice that the word anywhere is not part of the relative clause. It comes before the clause:
- she should have the right to sit anywhere [that she wanted to sit ___ ]
The locative prepositions where and wherever are different. They function as relative words. In the Original Poster's alternative sentence they take part in a fused relative construction:
- she should have the right to sit [where she wanted].
The verb SIT usually takes a locative complement. Locative complements usually begin with a preposition, or preposition phrase: sit up, sit down, sit here, sit on the sofa. In the fused relative example, there is no antecedent for the relative clause. The whole clause is a locative complement for the verb SIT. There is no other word that the fused relative clause describes.
This relative clause would have a gap in it, if wanted didn't 'stand in' for missing words. This gap represents the preposition where. We understand the sentence like this:
- she should have the right to sit [where she wanted to sit
The preposition wherever works in exactly the same way. Wherever emphasises that there is no limit to where Rosa Parks should be able to sit, if she wants to. But the meaning of the sentence is the same.
The fused relative clause version of the sentence has very different grammar from the anywhere version. In the fused relative, the word where is the antecedent for the gap but it is part of the relative clause. There is no other word which is the antecedent for where. This means that the clause with where is not changing the meaning of another word. The word anywhere is very different. It cannot function as the relative word in a relative clause. It can only be an antecedent for a relative word or relative clause.
Hope this is helpful!