For anyone who's interested in the mathematics of Influence, you do NOT get any for lifting an ally's sieged province. I tested this three bloody times last night, at great cost to the Turkish War Widows Association...
I have gained influence by lifting a seige (as Byz helping France-vs-English). It may have to do with your empire size, amount of influence, or some other mitigating factor.

The most fun I have had with allies was a battle between the Orthodox Allies and the Germano-Turk Allies. The Byz invaded Lithuania, seiged the rebels in the tower and got hit by rebels, and were helped by Novgorod, but invaded by Turkey and Germany all at the same time. 5 armies, 3 sides, a total mess (my computer was stuttering so bad I couldn't give orders)