I have a theory which I think is very interesting, but I dont know if it holds water.
Zhuge Liang's brother, Chin, worked for Wu. Chin was not held in a very high view in Wu, but I feel that Liang and Chin could have been planning something against both of their leaders.
What I feel is that Chin and Liang may have wanted to ally Wu and Shu, not for the reason of mutual protection against Wei, but that if Shu or Wu were sucessful in an attack on Wei, and susequently destroyed it, then Shu and Wu would have been the two countries left.
When Wei is destoryed that leaves Liang in a very comfortable position- not only is there an old leader (Liu Bei) but there is a young incompetant one coming up (Liu Shan).
Upon Bei's death, Liang may have used the connection with his brother and the (hopefully) allied houses of Shu and Wu to take advantage of both countries- Chin could keep arguing to the Sun family that they are tied by marriage, therefore not to attack, and Liang could slowly plan his domination of Wu provinces, getting his plan ready while Wu suspects nothing.
Near the end of SGYY there is a part where Liang is sent to attack Wei, (after his above plans seem somewhat unsucessful) but Huang Hao tells Liu Shan that he is planning to break away into his own state, what if there was some truth in that? Liu Shan wasn't a great general or leader, so if Liang had decided to do so, he would have found it easy to take Shu, but he needed to have removed Wei in the North to have a lessened threat of Wu on his Southern borders.
The reason that I thought of this argument is the insistance of Guan Yu's marriage to an Wu princess, and Liang's parsuasive argument to Liu Bei that he could marry Sun Xiang Shang, knowing some plan is afoot, but double bluff them and still have a wife. Liang also fails to really teach Liu Bei's son in politics, and maybe this is purpousful too.
With Shu and Wu allied, Liang is in the most powerful position ever, and Chin would have been rewarded in position if he was sucessful with Guan Yu's marriage to Wu forces, therefore making the Zhuge family a force to contend with.... IF they can conquor Wei.