In FTM, you can only link persons:
Child to parent / family and vice versa
Spouse to spouse
So, you have to work within those parameters.
Now, what do you know? Do you know if the aunt or uncle is maternal one or paternal one? If you don't have the answer to that question, I suggest you put the aunt or uncle in your notes.
If you do know if the aunt or uncle is maternal or paternal uncle, just create a dummy parent for the two children who are siblings and connect the children to the dummy parents (or just father}. If the paternal side, you can just enter the paternal surname. If the maternal side, you'll have to be more creative with a name to use - like just plain "father" for the father.
So, say you have Jane in your database, with a father of John Smith.
You know that Susan is an paternal aunt of Jane - ie sister of John.
Enter "Smith" for the father of John Smith.
(at this point, you might want to add a wife named "wife" to Smith to make them a couple - that's up to you.)
Enter "Susan" as a daughter of "Smith"
Now, Susan and John should show as siblings and Susan as aunt to Jane.
If you don't have the necessary people in your database, there are multiple instances of where to enter them, both in the Family tab and the person tab of the People Workspace.
If the people are already in your database, you can attach and/or detach them at:
Main Menu > Persons > Attach/Detach