I don't think there's any solid rumor about it at this point.
It's more speculation based on these elements:
1)Rockstar/Take Two confirming that they have new game announcements to make soon.
2)RDR announcement rumored by multiple sources to have slipped at the last minute from the last Sony E3 conference.
3)Playstation Meeting is usually the biggest venue for Playstation related announcements, they usually announce both new consoles and big games during these conferences.
It would benefit both Sony and Rockstar to announce the game during such event, even if the game is multiplatform.
So the chance is there, if it actually happens who knows.
I agree with Elios. The speculation is pretty strong, yet there aren't even any solid rumors about a new Red Dead sequel.
Rockstar has a license to do whatever they want, whenever they want.