It’s nothing to do with incest – other than that Darrin and his girl (Jessica, I think) are drawn to look a bit alike. Batiuk is really drawing out their having sex, so some people have speculated that an incest storyline was afoot.
Really, though, they aren’t related. Lisa (with the cancer) is Darrin’s bio-mother. She got pregnant with him in high school. Not an abusive relationship, IIRC, but the dude did decamp abruptly when it became clear that Lisa was PG. He’s showed back up a couple of times (again, IIRC), but is always a loser.
[checking today’s strip] Well, it looks like they finally did it… maybe. Kind of ambiguous after all the build up. Anyway, I think the thing here is that Lisa’s letter to Darrin is probably back in the PO box and, all befuddled by S E X, Darrin won’t get around to checking it again anytime soon. Maybe not until after Lisa is dead, so she won’t get to meet him. Sounds like the kind of abuse which Batiuk loves to heap on her poor bald head.