Anyone may create a page, that is correct. However, every page needs to meet the Avatar Wiki standards in both language and content.
In my opinion, the "content requirement" is where you would run into trouble when creating a relationship page for Opal, as there is no substantial notable relationship content for her. A relationship page is not a mere recollection of all instances two characters interact. It is a description of the deeper emotional relationship characters have with one another and the reasoning behind it. So for Opal, you would possibly be able to make such a paragraph to describe her relationship with her mother, with Lin (if you were truly turning every sentence you have of her), and Bolin (sort of). However, in my opinion, that doesn't quality as being a needed page.
That said, I like to emphasize again that it is just my opinion. I wouldn't create it since I don't see ample relevant information that could be added. However, this is just one opinion and if you think that there is a need for this page and that you'll have enough information to make a quality page about it, then by all means, go for it.
We know her relationship with her family,Korra and boling. in the next episodes we will see her relationship with Tenzin and the airbenders , maybe we will se her relationship with the red lotus and other characters like Kya and Bumi.
also we may see her in book four . because of that I see that we should add a relationship page.
-sorry to butt in- Good idea, Mkamind. But I think it's too premature for us to create such page; it's speculative whether Opal will make a role in Book 4 or not, so let's stay with the current: no relationship page until it meets our wiki standard.
She doesn't have a relation with Korra, they barely interacted. All there is to say so far about Korra and Opal is "Korra stated that Opal was a natural at airbending while going through the basics with her." Not really a defining relationship if you ask me. Same with her interaction with most other characters.
I am not opposing the creation of the page, I am just of the opinion that at this moment, there is no need for such a page due to the lack of substantial information to report on. However, this can change in the future, but that's something for which we'll have to wait and see and I don't see the reason why we should create a sub-standard page now just because there might be relevant information released in the future. We create a page to group relevant information. We do not create a page in the off chance that relevant information may someday be released.
Not any characters with a truly significant and notable relationship. However, as said, this is just my opinion. If you think otherwise, then make the page as you don't need my or any else's permission.