No idea since I haven't seen past here either, but nothing else seems to be mentioned on the Fandom so it doesn't seem like it comes up again. Though it's such a minor thing I also guess it's possible no one's added it so far.
Hmmm. Apparently she actually says it (Deidara-nii) once in the manga too according to the wiki, Naruto chapter 461, page 14. So it's not just a filler choice.
Looks like it could be either since Japanese uses it both ways, which doesn't really help.
Seems like people theorize that she says it either because they are both from the same village or maybe extended relatives.