Tween

In animate -I’m trying to create a graphic tween. The idea is to see pages turning. I have a key frames for each graphic - of a flat page to move to a half turned page to move to the other side of the book -half way, then flatten on the left side, so a flat page. The problem is the half on right to half on left. I cannot get the program to keep the spine of the page fixed so it “flips” over. Can you give me some guidance? I get a flip result if i have a stroke around each page graphic, but the stroke twists poorly in the motion. 

-mark