I mean, honestly? No.
I've been earnestly looking for reasons not to, but after a pretty frank conversation earlier this week I'm pretty much tapped.
The common concerns:
Procreation: Twin Girls, not a concern
Power Dynamics and Manipulation: We're both Mentally Ill (Clearly) and entirely codependent anyway.
Damage to external relationships: I addressed it with our two closest friends, both of whom responded that they actually had already assumed we were fucking but wanted to be polite and not mention it.
As I said, I'm open to suggestions here before I make the plunge, because as Confucius said: "You can't unlick your sister's butthole."