One other possibility, though also very rare. is near-identical twins. If twins have the same genetics (in other words, they’re identical twins) but have intersex conditions that result in different reproductive development it’s conceivable that they could reproduce with each other.

Yeah, aren’t there occasional cases of have XY (male) and X0 (female, Turner’s syndrome) twins? That seems like the closest to OP’s scenario, and yes, it would definitely be incest.