Kate and the ex are hella incest

thats the screwed up directors cut edition. no one wants that version.
the original plays it all just fine without that silly part of it. it was kinda stupid to put that whole connection part back in.

She's apparently his great-great-grandmother, right? That's nowhere near 100% blood related. You're only 50% related to your mother (and the other 50% related to your father). Then you're 25% related to your grandmother, 12.5% related to your great-grandmother, and 6.25% related to your great-great-grandmother.

That makes it comparable to some sort of distant cousin, the kind that it's perfectly legal to marry. (I would be more specific, but it's too late at night for me to figure out the math.) You could easily get to know such a person, fall in love, get married, and never even know that they were some sort of distant relative unless you were a genealogy buff.

So there's really no reason to worry about Kate and her ex.

They were supposed to be a romantic couple in the original script. The director caught on to the same thing you did and took that part out.