I tried to explain it, but you didn't listen.
I have worked with a lot of incest families due to my actual profession, so I know something about it.
There is a slight difference between biology and legal mumbo-jumbo. Law does not define incest, biology does. Read on...:
Incest is, yes, one to two steps away in a direct line. An aunt/uncle isn't in a direct line, since they are not of the same blood as you (your parent's sibling doesn't share close enough DNA for it to actually be inbreeding). That is why sex with an aunt or cousin is not biologically regarded as incest.
An aunt is not two steps away in your direct blood line. A grandparent is. An aunt is two steps away, but in an indirect line which makes it legal. A first cousin is three steps away.
Again, incest if: child, sibling, parent, grandchild or grandparent.
You can, in most countries, marry your niece, cousin, aunt or uncle. That is why it is not incest if you have sex with them (and sex with them is legal in all western cultures. In very few prudish countries, like the US, you cannot marry your aunt or cousin even though it is legal to have sex with them, but you can in most other western countries).
And here I didn't even talk about aunts of marriage - an aunt who married your parent's sibling. There is no blood relation and absolutely no discussion that this could never be regarded as incest. I didn't include this originally because in this game, the aunt is MC's mother's sister (if she actually is his mother, but that's what he thinks at least).
On a funny note, many games try to make it "okay" for stepsiblings to have sex. That is actually, for some reason, illegal in most western countries, even if the stepsiblings don't share any DNA. This is legal stuff: the family connection is too close.