Dating Your Uncle's Brother's Wife's Niece

OK, you say it's your father's brother's wife's neice.  I trust then that we can safely say since your father's brother is your uncle, then this is your uncle's wife's neice, correct?  And, since your uncle's wife would be your aunt, then it's your aunt's neice, right? 

Now it gets interesting--this neice could be the daughter of your aunt's brother and sister-in-law OR your aunt's sister and brother-in-law.  Either way, there is no direct bloodline that would link you to this neice, as she is the result of a union between your blood uncle's spouse and somebody else outside both your uncle's (hence, your) and his wife's (your aunt's) families.

You might want to check local laws to determine any issues of legality as to whom you are able to date!

By marriage, Seun.  Or, if one would prefer, "aunt-in-law."