Some scattered thoughts. Take them as you will:
-- maybe a different time of day would be better? There's something about swimming/chlorine that's kind of tiring... or maybe she's hungry.. or, or... Is there time early in the morning before swimming? Or maybe last thing at night (so that she feels like she's getting an extra 20 minutes before she has to go to bed)?
-- rotate "reading" activities. maybe try pieces she doesn't know, so the incentive is she reads a bar, then she plays it and finds out what it sounds like, then reads the next bar, plays it, etc. until she's done the whole piece. and play games like "guess that note" where you play a note while she's not looking at the piano and she has to guess what that note is (and maybe write it down?) Or give her sections of pieces that she already knows and have her read them so that she can figure out what piece it is. (Does that make sense? Like the third bar of Twinkle, Twinkle, Little Star or something). Mix it up, and make it seem more like play than work.
-- And finally, go easy on herself and you. There's no reason she has to be a sight reader by August. Hopefully, you'll have a lifelong music lover, and one day she'll be reading the most astounding pieces. She can get to that place from insane amounts of exposure, sprinkled with a bit of formal teaching. (This reminds me of my typing skills. Haven't needed to look at the keyboard in well over a decade; wasn't taught that -- just exposure.)
G'luck and remember to have fun!
