Ok, if you’re an Ó hAonghusa* and your nephew is a MacUltaigh**, you don’t get to be a Titi. You can, if you really wanted, be an aintín. But in that case, since it’s a loanword, you might as well be an aunt — which is, by the way, what I would be if my siblings were to reproduce. However, the only nephews I currently have (thank goodness) are the offspring of My Greek-o-Rican Fella’s equally Greek-o-Rican brother. Hence: Titi Gill.
Concerning the theoretical progeny of my blood kin, I would — in addition to the aforementioned terms — answer to piuthar-màthar/athar, modryb, tante, and (nagy)néni. Maybe some others, as there are some unexplained genes in our familial pool.