Heheh i have the exact same setup pally tank priest healer 3x mage dps <3 <3 dunno if it helps but i dps with:

#showtooltip
/cast [exists,harm,nodead] fireball
/stopmacro [exists,harm,nodead]
/assist Party1
/cast [exists,harm,nodead] fireball

If you don't have a target... or current is dead then it will pickup your current target (party1 is the pally tank) to sheep i have two macros... first to grab focus:

/assist party1
/focus [target=focus]
/p --== Focus is on "%f" ==--

(on my main i have three keybinds one for each mage furthermore each key also sets a raid icon - the luckycharms symbols so i can verify there wasn't any lag and each have the right focus)

then to sheep...

#ShowTooltip
/cast [target=focus] Polymorph
/p --== Sheeping "%f" ==--
/target Party1

i added the retarget at the end just as a catch22 if my mages ever were attempting to dps the wrong target (stuck on someone in the distance cause i hit the firball key on accident etc) so i can just spam a fake sheep on each mage to get them to reset and assist me again (you prob could put this in a seperate macro all together but it hasn't been bad for me yet and i try to minimize the ammount of buttons i need