It depends. Aunt/Nephew is less Taboo in general, but could be handled in some ways that are just as Taboo as mother/son.
I think the main difference is that people are willing to believe that Aunt/Nephew pairings happen more than Mother/Son.
I completely disagree that a mother has to be loving, in fact, there's a lot of asshole mothers out there, and its a lot more believable that an emotionally damaged mother commits incest with her son than one thats your perfect TV mom that spreads her legs for her son.
I think you'll get a lot more attention with a mom/son game than aunt/nephew game as a lot of 'teen boys' probably would find the mother angle a lot more taboo/erotic.
However, I think you could do a lot with an aunt as well, and its never really focused on much in games that well.
1) Pervert aunt that loves younger guys (corrputs MC)
2) Pervert aunt that already has incest experience with MC's dad (she's dad's sis)
3) Aunt that is suddenly given custody of hormone crazed teen boy and has no idea how to draw lines/punish the boy
Some mom scenarios that are rarely covered:
1) Mom hates the MC and tries to ruin his relationships eventually going as far as seducing him away from his girlfriends. This would be a gradual twist where the Mom has no sexual designs on her son, but uses sex to creep out his friends/girlfriends, dropping hints that she'd like to be part of a 3some, or that her son is a pervert, ect - and then slowly realizes that she'd actually enjoy the MC being sexually deviant.
2) Mom is in the middle of a mental breakdown and uses sex as a way of keeping men in her life, or sex to avoid her problems. The MC can be a little bit of a shitbag or in trouble at school and the mom uses sex to keep him in line.
3) Mom has previous incest experience and is actually fine with incest (its her kink) and she's been dropping hints on MC but he's maybe too chicken to go for it.
Mother or Aunt is probably fine, but as Thailf stated, with the Aunt you'd probably need to make the Aunt something more than the 'girl next door' to get people interested in it.