Responses
The definition of incest is the sexual relationship with closely related ppl, it is never specifically stated to be with only certain family members. Since the aunt in this case is her mom’s sister and not a distant relative, her son is closely related enough to be considered incest. But though relationships with first cousins are widely considered incest, it is still accepted in some cultures.
I just wanna ask, if your aunt's son have a romantic feelings towards you, is that called incest too?
(Ps: your aunt is your mom's little sister)