– While the plan to stop what they set in motion by getting DeWitt to Columbia was mostly instigated by Robert both were also slightly pissed that they were betrayed by Comstock and that it could have cost each others life. Obviously they do not mind the state they are in now but no one could have known it would end this way and not by them dying or something similar. The thought that it could have killed their ‘sibling’ after working so hard to be together weights more than their own possible death.

– The first messages they were able to send to each other were very cryptic and often parts of them were lost or messed up. This only piqued their interest in the other.

– The universe in which they are is the only in which they can be together and also live the life they want with funds for their studies and experiments while being seen as siblings by the general public. If there were other possibilities they wouldn’t help Comstock to the extent they did and switch to Roberts reality instead of to Rosalinds. 

– To a certain extend Robert and even Rosalind actually cared about DeWitt for more than just rectifying the course the universe. While still looking at it in a scientific manner (especially how there are endless worlds from which he can be taken) they do want him to succeed. That’s why they keep giving him hints and actively help out in the Background. Robert, while not acknowledging it, also feels guilty for taking away his child, since both Luteces grew as humans while being together.

– When getting adjusted to his new reality Robert made up different kind of explanations for this new surroundings and especially Rosalind. They got the idea to refer to each other as siblings from this phase but it also included Rosalind being his wife, assistant or even a mirror image.