system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why don't I find my mom and sisters sexually attractive?
gpt
It's not really clear to be honest.

Westermarck, an anthropologist, proposed that we are evolutionarily "trained" to avoid sexual contact with people we are very close to for the early part of our life (brothers, sisters, parents, etc.). 

The idea is that evolution chooses people who show this aversion/disgust towards sex with those they are close to early in life because reproduction between close family members leads to reduced genetic diversity and (eventually) accumulation of bad mutations. 

He proposed that this "Westermarck effect" could be proven because even non-related children who grow up together tend to show similar repulsion towards sex with each other (think of a close family friend you've known since childhood who you'd never want to have sex with because they're "like a brother/sister to you"). Vice-versa, siblings separated at birth who meet much later in life might find themselves sexually attracted to each other despite being related (see, news stories about couples who later find out they're related due to DNA testing, shock horror!).

HOWEVER: There are plenty of flaws in this theory however, and plenty of studies which have come out to oppose its findings. Other studies have found that people are attracted to people with more similar features to their own, which seems to contradict this theory. Long story short, science ain't super clear, but it's likely to be a combination of cultural and behavioural factors, tied to evolution.