Motherly Kogal ☆ Knockout

I think that's what's implied that she's a cousin that was married into the family from her mom. Her mom is probably the wife to his legit uncle. If im wrong oh well

You are correct. Any sexual act involving blood relatives counts as incest.
"Inseki" applies only to unrelated persons who have become "relatives"
through marriage, like stepparents, stepchildren, step-siblings, etc.

(It's worth noting that, in the real world, the incest taboo tends to weaken
as the blood connection becomes more tenuous. For example, you could
probably marry a fourth or fifth cousin, and no one would make a fuss.}

For some reason, Fakku tags sex between
cousins as "inseki" when it's clearly incest.