Bathroom Light / Fan Switches - Changing Switch Controls
So the short answer is you can take one of the wires out of the nut and swap it with the red wire of the fan, and that will do what you want. 

The medium answer is you could do what Harper suggested and replace the single switch with a double switch and control all three independently. You would take the bottom wire from the switch and place it in the brass terminal, and one each of the 2 wires going out to the lights in the 2 silver terminals.

The long answer is, your drawing only shows "hot wires". They should all be black or red. My guess is that nut of 4 wires is a hot coming in, a hot going out to the next outlet or switch, and the 2 pigtails that "energize" the switches. The neutrals are either at the fixtures themselves and were never routed in your switch box, or what you think are the ground wires are your neutral (are they bare wires or white?). The ground being the metal clad sheathing of the wire that is common in older homes.