How many adolescent girls have asked the same question? Supergirl realizes that a large part of Power Girl's power is what she does to men. Stupid "S" logo! Without it, Supergirl could have her own (smaller) cleavage window...
If I'm not mistaken wasn't power girl/woman the super girl from the future that was related to super from relations on earth? Super girl came to earth via space, they aren't even the same character.
powergirl tends to get rewritten at the drop of a cape. She's either an older supergirl from an AU or supergirl who aged on here way to earth and also from an AU.
I love how every so often there's an attempt to give her a new, less revealing costume. But then something happens to burn a brand-new hole in her chest piece. The fates demand cleavage.
BVV
Then again, that was from the Justice League cartoon show, where she was a clone, so eh.