is the revulsion to immediate family incest biological, or we're somehow subtly taught in childhood via our elders?

I'm guessing that the feeling is social, rather than biological. Other mammals (and "lower" animals) have no such compunction. In my own case, I've seen young men almost drool down their shirts when looking at my sister and try to get close to her. Although I acknowledge that she was and is sexually attractive, I have never been attracted to her in that way.

The Westermarck effect, also known as reverse sexual imprinting, is a psychological hypothesis that people who live in close domestic proximity during the first few years of their lives become desensitized to sexual attraction. When proximity during this critical period does not occur—for example, where a brother and sister are brought up separately, never meeting one another—they may find one another highly sexually attractive when they meet as adults or adolescents, according to the hypothesis of genetic sexual attraction. This supports the theory that the populations that appear to exhibit the hypothetical Westermarck effect became predominant because of the deleterious effects of inbreeding on those that did not.

nope - it's just familiarity breeds contempt......

It's not "immediate family," it's "someone we grew up with."  There are just way too many stories of families broken up when kids were infants, then grew up apart, and when grown married each other without knowing of the biological relationship.

I don't know what the cutoff point is for age or amount of growing up together.  I know of a couple who met in 7th grade junior high;  maybe they didn't spend enough time together in junior high to trigger the "yuck" factor, or maybe that's too late in childhood / too close to puberty for "yuck" to happen.

Yes, social indoctrination of cultural taboo.

Many ancient cultures

and probably still some modern ones

had no such cultural taboo

and it was quite common for siblings to marry.

Famous example: the last king of Hawaii, who, when he and his sister-spouse were Christianized, felt required to live apart from each other for their remaining years, reportedly both extremely sad about this for the remainder of their lives.

In the Bible

- Abraham married his half-sister

- the father of Moses married his father's sister (i.e. his aunt)

- - - Yep, Moses was his father's son...and first cousin

In history, SEVERAL of the Ptolemaic kings were married to a sister...or two!

(Clearly no such taboo in THAT culture!)

Probably a bit of both. In the ancient world people often married close relatives. This happened among the Pharaohs and resulted in genetic illnesses. Abraham married his half-sister. It wasn't until Moses and the Exodus that the laws against incest were written in the Torah.

In the more recent past because most people didn't travel much they often married cousins and other close relatives resulting in inbreeding. The same with the Hapsburgs royal line which had a lot of inbreeding.