“All right, I’ll swing 2 in the air with this guy, and 3 with this one.”
“Okay, I’ll draw 2 cards from my Sphinx.”
Does that sound right? Well, it should! Windreader Sphinx doesn’t much care where the winged swings are coming from, or where they’re aimed. The ability on the Sphinx is “Whenever a creature with flying attacks, you may draw a card”. Not a creature you control, not when a creature with flying attacks an opponent. Just… whenever a flier attacks. Your guys attacking draws you cards, flying creatures attacking YOU draws you cards, flying creatures attacking an opponent in a multiplayer game draws you cards. Flying creatures attacking planeswalkers will still draw you cards, too.
The only criteria for the trigger is that the creature with flying must have been declared as an attacker, and it must have had flying at the time it was declared as attacking. You can’t use something like Zephyr Charge after the attack to draw a card, nor will removing flying from a creature in response to the trigger stop the draw; “Do you have flying?” is only checked/only matters once. Also, the creature must actually be declared as an attacker; the angel token that hangs around with Geist of Saint Traft isn’t, she’s just put into play already attacking. Because of that, she won’t draw you any cards off of Sphinx.